To be as flat as a pancake, to be as flat as an ironing board
Contexto
Flunder: platt sein wie eine Flunder -sal selten- 1.3. s. o/s. th. Is as flat as a board, 2. s.th. Is as flat as a pancake -coll-, s.th. Is as flat as a flounder -coll-, 3. s.o. Is flat-chested, 4. to be flabbergasted -coll-, to be dumbfounded n. […] Die Barbara macht mich nicht sonderlich an, die ist ja platt wie eine Flunder.