(as) flat as a board

1. slang Physically squashed or flattened. I forgot that I had a muffin in my bag, and it's flat as a board now. My model of a volcano is as flat as a board now that you've sat on it!

2. rude slang Having small breasts. I heard that Amanda's actually flat as a board and just wears push-up bras all the time.

*flat as a board

and *flat as a pancake

Cliché very flat. (Also used to describe someone's chest or abdomen, referring to well-developed abdominal muscles or small or nearly absent breasts or pectoral muscles-in either sex. *Also: as ~.) Jane was flat as a board until she was sixteen, when she suddenly blossomed. Lucy can mash an aluminum can flat as a pancake with one blow from her heel.