Green’s Dictionary of Slang

foolish, gullible.

1942		Berrey & Van den Bark Amer. Thes. Sl.
1946		Mezzrow & Wolfe Really the Blues 139: Some hammy clowning on the bandstand to tickle the simpy customers.
2005		J. Stahl I, Fatty 222: This may sound simpy after what he did to me, but [...] Fred wasn’t a bad guy.