Noun
(regional, slang) A flatfish.
(Australia, slang) A flathead (fish).
(slang) A flat tire/tyre.
(archaic, US, slang) A policeman.
(slang) A flathead V8 engine.
(slang) A hot rod fitted with a flathead V8 engine.
(slang) Alternative form of flattie (“operator of crooking gambling operation”)
Source: en.wiktionary.org