flat as a board
(as) flat as a board
1. 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. euphemism, rude Having small breasts. I heard that Amanda's actually flat as a board and just wears push-up bras all the time.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
*flat as a boardand *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.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.