Parent-Fu
So, this old dude waltzes in and beats the living hell out of someone or something, and you already know who they are. They are, in fact, the main character's Father. Often times, he'll be beating up the main character, unless the protagonist is female, largely because he wouldn't hit his dear sweet daughter. The guy doesn't have any reason to be so powerful, and it doesn't necessarily mean the hero isn't fighting back because of filial loyalty. Basically, he knows Parent-Fu. Because he's your daddy, he's more powerful than you, which makes this a subtrope of Authority Equals Asskicking.
It should be noted that this encompasses perception. Just because the author explains everything later, doesn't change the fact that they were running on Parent-Fu at the time. Compare with knowing mortal combat for an equally flimsy justification for a character's power, contrast with most other forms of -Fu in that the character need not have any training.
Anime and Manga
- If this troper is not mistaken, in Bleach, Ichigo's Father is able to take down Grand Fisher, seemingly for no other reason than being the father of such a Badass. If it had already been explained that he's a Soul Reaper, then no it wasn't, and this example should be removed.
- It didn't explain it until the point he was already fighting, nor did it explain how powerful he was.
- Also in Ranma ½, not quite a father but a grandfather - amnesiac Shinnosuke's grandfather always uses the "Megaton Punch" whenever his son forgets anything important.
Film
- Pootie Tang's Dead Father in Pootie Tang, who was demonstrated to be able to strike anywhere at any time with his belt, with lightning speed. This was temporarily explained as a magic belt, but was later explained as him being simply that Badass. Further, he was only demonstrated being this way with Pootie Tang, which was a very real case of being powerful because he was Daddy.
Western Animation
- Robert "Granddad" Freeman from The Boondocks, who demonstrates a remarkable skill with the belt and is at least Huey's (who has proven himself ridiculously capable for his age) equal in combat, for no clearly stated reason.
Other
- Comedian Sinbad had a routine where he described his mother's particular ability to punish him, including spanking him so hard his butt fell off, then launching her hand Rocket Punch-style when he ran. Which, of course, lead to the immortal line "It's your momma's hand! Drop your butt and run!"