Green’s Dictionary of Slang

(US) a stupid person.

1941		in A.T. Horman Buck Privates [film script] Go on, maroon, get a seat [HDAS].
1957		(con. 1943) A. Myrer Big War 222: Don’t be a bunch of marrones.
1967–8		Baker et al. CUSS 155: Maroon A person without much social or academic ability.
1992		N. Cohn Heart of the World 225: ‘What a maroon,’ she said.
1997		E. Little Another Day in Paradise 80: These guys are like neo-Nazi lightweight hoods, fucking maroons.