Flat as a board （ぺちゃぱい）

He Says:
pecha pai (pronounced "pie") （ぺちゃぱい）=flat chested
What kind of pie do you imagine "pecha pie"?
New delicous pie?No!umm,but sometimes yes.
Pecha pie is flat boob.
So like a men breast, girls who have flat boob, we would call that pecha pie.
It's a little bit hard to explain why we say so. I think a pie is usually big because it has many cream or fruits inside. But when the pie is crushed or fallen, it can be flat. Pecha is an onomatopoeia word--the sound something makes when being crushed or made flat. So pecha pie is like no volume pie, it's a flat boob.
Many American guys seem to like Japanese girls. But Jp girls have usually pecha pie. It's hard to find a big boobed Jp girl.because the average of jp girls bust is said 82cm (about 32 inches).

She Says:
His description sums it up.
Japanese uses a lot of onomatopoeia words which are incredibly difficult for foreigners to master. You'd think that you'd just make the sound that something makes...but you actually have to know how Japanese people will say it. For example, to American ears the onomatopoeia words "kachung" and "darung" might pretty much sound the same. But to a Japanese person, they are totally different.
I'd always use the wrong onomatopoeia word and have people stare at me like I'm crazy.

The next word is...ポリコウ。