貧乳って英語でなんて言うの？

You can say, "I'm flat." or, "I'm small breasted." Both would work! A fun little saying we have is "I'm a member of the itty bitty titty committee." Which is a fun way to say your flat chested.

The chest is the area at the front of the female or male body that covers your rib cage.

Well, the first description is probably the kindest in that it does not offer a critical opinion. The second description assumes the breasts have not developed correctly and so are small. The third opinion describes the breasts as, 'nothing', so the chest would be similar to that of a boy or man. Both the second and third descriptions are derogatory and would not be used directly, but rather, only in third party conversations. 'Flat chested' is also not appropriate for use directly to someone as it seems critical and rude. "Jane is rather flat chested for a woman of her age."

The breast or chest area is the top half of the body that covers where your ribs are which can be on a man or woman. You could say 'I'm flat chested' or I have small breasts meaning you are not very big in the breast/chest department for a woman both are a nicer way to say this.

There are some more polite ways to describe someone who has a flat chest, when describing in a sensitive situation, it's best to try to remain as respectful as possible by saying phrases like : flat chested, small breasted or flat breasted.

Flat chested is a more common expression is not considered crude but it is also not a formal or medical term. Flat chested is well known and used more commonly for everyday English. The word "breasts" is used when speaking more formally. Words such as: titties, boobies, and tits are considered slang words and are not appropriate to use in many public settings.