flat as a board
*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.