Flashback to Catchphrase

Young Ruri: Excuse me. What is the meaning of this word? "Baka"?
Teacher: Um... That's a word that you shouldn't use in polite conversation.

Young Ruri: Baka...

—Martian Successor Nadesico

A major character has some sort of Catch Phrase or other profound statement which defines his/her character. A Flashback to Catchphrase is a Flash Back which takes place before the series chronology (or at least before this major character was introduced) which shows the moment in which the phrase came into their vernacular.

Sometimes they come up with it themselves and automatically remember it due it occurring at a pivotal moment in their lives. Other times it's depicted in more of a throwaway-moment where they just like the way it sounds.

In some situations it's another character, sometimes appearing only in this flashback, giving some sage advice which forms the basis for said catchphrase or profound sentiment.

Some examples even show that hitherto, the character had been engaging in the opposite fashion of their familiar, later, catch-phrase spouting self before learning their signature words!

Compare Ironic Echo, Meaningful Echo.

Examples of Flashback to Catchphrase include:

Anime and Manga

At the beginning of Rurouni Kenshin, Kenshin says his motto that "a sword is a weapon, and kenjutsu is the art of killing". Later we learn this phrase comes from his sensei Hiko.
Early on in Naruto, Kakashi says "Ninjas who break the rules are scum. But ninjas who abandon their friends are worse than scum." Later, there's a flash back in which, on a mission, Kakashi had been attempting to follow the rules strictly to the letter at the expense of a captured teammate, and his other teammate pretty much says this.
- On paper, though this looks like an asspull, as the flashback takes place in the Shippuden series, which takes place after a 3-year time skip, which is also after the 200+ episodes of the original series...
  - In the manga though, the flashback happened much earlier - they just didn't animate it until Shippuden, for some reason.
During Martian Successor Nadesico's origin episode for Ruri, among the other things she experienced in the facility where she was raised, we learn that she was inexplicably drawn to the word "Baka" the moment she saw it, as shown in the page quote. By this point the viewer knows well that she latched onto it for the rest of her life.
Several times in Gundam Wing, Heero Yuy states "The best way to live your life is by acting on your emotions." The prequel manga Episode Zero shows that he picked up the phrase from his mentor-slash-father figure Odin Lowe and it's one of the last things he says to 8-year-old Heero before dying.

Film

Interestingly inverted in Back to The Future, with Doc's "If you put your mind to it, you can accomplish anything!" being repeated by Marty to his 17-year-old father while in 1955, to discover back in the future that George has adopted it as his motto.
- Another inversion, Older Doc's use of the word "heavy".
Star Wars: The creation of C3PO's name comes from the sequels.

Literature

In the Discworld novel Night Watch, a Time Traveling Vimes, buying a pie from a young CMOT Dibbler, says "And that's cutting your own throat, eh?". While the young salesman is initially confused he later decides he likes the sound of it, supposedly creating a Stable Time Loop (although not really, as the man Vimes is pretending to be was a real person in the original timeline. On the other hand, who knows?
In Honor Harrington, the main character's Catch Phrase is "Let's be about it." On her midshipwoman's cruise, guess what her first commanding officer seems to say at every opportunity?
A number of Harry Dresden's spell phrases and sayings are this. Being Jim Butcher's works, these almost always overlap with Stealth Pun. Fliccum Bicus? Harry tried to cheat at a fire-starting spell. [DuMorne] told Harry that he wouldn't always be able to "flick his bic".
- Happens a ton in Ghost Story, though it's justified at length as part of being a ghost.

Live-Action TV

Prison Break A number of character traits for the characters are revealed through a 1st season episode that had a lot of flashbacks in it.
Frasier Crane's catchphrase was "I'm listening"—revealed to be in a flashback episode to inadvertently come from Frasier's father Martin, who had been trying to listen to a football game while his son was talking.
"Two Cathedrals", the second-season finale of The West Wing used this trope as the basis of the episode. President Jed Bartlet is prompted by the death of his secretary, Mrs. Landingham to flash back to when he met her as a young boy. His mannerisms in the past (putting his hands in his pockets and smiling when he's made up his mind to do something other people will not like) are referenced to foreshadow his exact actions at the press conference he calls to announce he will, in fact, be running for re-election with a relapsing/remitting course of multiple sclerosis.
One Halloween episode of That '70s Show has a flashback to the day Red first uttered his catchphrase:

Red: He's an ass. And he's dumb. He's a... (spotlight and Cherubic Choir) ...dumbass.

Heroes does this when Hiro meets himself as a child.
Known as it is for flashbacks, a season 3 episode of Lost features one of Desmonds flashbacks to his time in a monastery, revealing where he got his curious habit of calling people "Brother". He picked it up from the monks.
- Also, Locke's flashback in season 4 gives us a teenaged John uttering the phrase "Don't tell me what I can't do!" that would later define his character.
  - Prior to that episode, another episode in which we see how Locke was paralyzed featured a doctor telling Locke that he shouldn't tell himself what he can't do. It was mistakenly assumed this was where he picked up the catchphrase until the above episode aired.
- One of the straightest examples of this trope is Jack's infamous "catchphrase" (which he says twice in the whole series) of "WE HAVE TO GO BAAAACK!" In a season 5 flashback, we learn Locke first said this to Jack.
- Hurley's mantra "you make your own luck" has interesting origins. He heard it from both his father and Martha Toomey before using it on the Island.
  - The numbers themselves qualify, to some degree. In "This Place is Death" we get a time travel flashback to 1988, and we get to hear the mysterious transmission of the numbers from the Island. This was also heard by Sam Toomey and Leonard Simms, which is how it got to Hurley. And since the transmission sounds a bit like Hurley's voice...
    - Then in season 6 it's gently suggested that the ultimate origin of the numbers was Jacob's pointless affinity for numbers in general.
In How I Met Your Mother, it's revealed that womaniser Barney used to be a hippie, and that his look, attitude and catchphrases all come from the guy who stole his girlfriend off him.
- Except "Suit Up!", which came from an advertisement.
Used to some effect in Torchwood. Captain Jack Harkness says in the introductory narration "The 21st century is when everything changes, and you gotta be ready." In the Series 2 episode "Fragments", there's a flashback to 1999 in Jack's past, where Jack comes back from a mission and his former team leader, Alex, has killed the rest of the team. Alex, just before killing himself, says, "Everything changes in the 21st century. We aren't ready."
Though it isn't a flashback, Stargate SG-1 had a scene where Jacob Carter used his daughter's catchphrase "Holy Hannah", showing us where she picked it up.
In Scrubs, J.D.'s moussed-up hair was apparently due to a suggestion by his college buddy, Spencer.

(in a flashback)
Spencer: Why don't you just mousse the crap out of it -- straight up!
(end flashback)
J.D.: My life changed that day.

Video Games

In Tales of Symphonia: Dawn of the New World, Emil has a flashback to when Richter says, "Courage is the magic that turns dreams into reality." It then becomes Emil's own catchphrase.
- Note that the player sees when Richter says this in the first place. And then when we see the flashback...again and again and...
Axel is known far and wide for his catchphrase -- "Got it memorized?"—almost always used after introducing himself. In the recent prequel, though, we see just where it came from. As a child, he decided that he'd be metaphorically immortal if everyone remembers him always.
"After all... isn't that what a gentleman does?" "Luckily for me, every puzzle... has an answer."

Web Original

Order of the Stick's Big Bad, Xykon, adopted his name as a teenager after meeting a thinly-veiled parody of Professor Charles Xavier from the X-Men?specifically because he thinks names that start with "X" sound cooler.
- Also, Haley's distaste for her lime green Boots of Speed (mentioned several times throughout the strip) is shown to be the result of Crystal mocking them on their first meeting.
"Memory is the key." in Red vs. Blue. So much that Caboose lampshades this ("Aren't we done with this already?")

Western Animation

In Phineas and Ferb, Harmless Villain Dr. Doofenshmirtz often expresses the intention to take over "the entire Tri-State Area!" The episode "What Do It Do?" shows us a flashback to a date he went on in high school with the future Linda Flynn, and when he tells her he plans to take over the world, she suggests that he start small.
In The Simpsons episode Dangerous Curves Ahead, we get a flashback to one of Homer's many catchphrases: When Homer and Marge are dating, they meet the newly-wed Flanderses for the first time. Homer initially likes Ned, but when they stop at a motel, Neddy says an unmarried couple can't share a room.

Homer: I never thought I'd say this, but, stupid Flanders.

- Another flashback episode has Homer and Marge meet as children, but not realising it because they used pseudonyms. When Marge calls asking for Homer's obviously fake rockstar mashup name, Moe responds with the string of threats for which he has become known. He the turns to the camera and says "And that's the origin of that thing". The same scene also has Patty and Selma decide to start smoking.
  - Even though Patty and Selma already took up smoking in a completely different flashback in an earlier episode.
- Also in "Lisa's Sax", it shows a five-year old Bart (who sees school as a living nightmare) came to get attention from the other students by acting out. When Principal Skinner came to reprimand him for this he replied "Eat my shorts!"
- In "Lisa's First Word", it is revealed that Bart’s first word was actually his catchphrase "Ay caramba!"
In the Whole-Episode Flashback of Teen Titans where we finally get to see them meet, ("Go!")Cyborg fires his sonic blaster for the first time.

Cyborg: I'm only going to say this once; booyah.

- And you can't forget that in the same episode it shows why Beast Boy says "dude" and the beginning of Robin's catchphrase.
Beetlejuice: "There comes a time, Babes, when you do things you've never done. When you say things you've never said. And so, for the first time ever, I said those magic words: 'It's SHOWTIME!'"