Green’s Dictionary of Slang

1. (US) used with a variety of nouns to imply obsessiveness.

1926		J. Black You Can’t Win (2000) 74: He’s a railroad bull and he’s ‘bum simple’ — simple-minded on the subject of killing bums. [Ibid.] 158: He went out on the road ‘bull simple,’ simple on the subject of shooting policemen.
1928		J. Callahan Man’s Grim Justice 138: The old timers [...] said we were ‘cow-simple’. [Ibid.] 139: If y’ git cow-simple, kid, y’ can’t associate with any of the old-timers.
1935		A.J. Pollock Und. Speaks 27/1: Crime simple, one who constantly thinks of crime.
1953		T. Runyon In For Life 245: Shakedown-simple, he couldn’t pass a drawer without checking it.

2. as sfx implying ignorant.

1999

J. Ridley Love Is a Racket 73: Why is it only guys who are money-simple ever fall into dough?