Green’s Dictionary of Slang

1. (US prison) a fool, a dullard.

1950

Goldin et al. DAUL 162/1: Popcorn. A fool; a person mentally deranged or feeble-minded.

2. one with a legitimate job, rather than a criminal or a confidence man.

1953

‘William Lee’ Junkie (1966) 157: Pop Corn . . . Someone with a legitimate job, as opposed to a ‘hustler’ or thief.

3. anything mediocre and lightweight.

2001

Guardian Rev. 11 May 13: It’s a quiet, complex piece, problematic for those addicted to popcorn.

4. (W.I. drugs) marijuana.

2007

Marijuana Growing Forum Apr. 🌐 There’s this weed real popular over here called ‘Popcorn’ or Corn for short.

5. see popcorn pimp n.