"Well, excuse us for being flat!"—Etna and Flonne, Disgaea
This might actually bother her if she is already considered childish and prone to tantrums, so the sore spot becomes endearing. Occasionally, she is proud of that flat chest as a way to appeal to a certain demographic of guys, although real fans in practice make a distinction between merely being flat-chested and actually being underage. Nonetheless, it is very useful in creating a character who looks younger than she actually is.
Also known as "Delicious Flat Chest", or DFC, a term apparently coined by Image Boards.
Now has its own song.
Note: This is a fan-speak term, not a trope (although the term is used in dialogue in Seitokai no Ichizon). Do not use it as such. Doesn't mean it can't still be used, but just not as a trope. Much like People Sit on Chairs, this isn't a storytelling device.