c. 1600, "with flat feet;" see flat (adj.) + foot (n.). Meaning "unprepared" is from 1912, U.S. baseball slang, on notion of "not on one's toes;" earlier in U.S. colloquial adverbial use it meant "straightforwardly, downright, resolute" (1828), from notion of "standing firmly."
1. Early American weathervanes were most often cut from flat wooden boards.
2. Warm weather has attracted the flat fish close to shore.
3. Two years later he is flat broke and on the dole.
4. She had small feet and hands and a flat, round face.
5. I lay the painting flat to stop the wet paint running.