Green’s Dictionary of Slang

popcorn adj.

[the banality of the foodstuff]

1. (orig. US Und.) foolish, slow-witted, lightweight, second-rate.

[US]F. Harris ‘Gulmore, the Boss’ in Elder Conklin & Other Stories (1895) 173: What he said was popcorn; but it took with the Mugwumps.
[US]W.J. Kountz Billy Baxter’s Letters 58: He is tight-ribbed and popcorn.
[US]News & Courier (Charleston, SC) 14 Apr. 18/1: Chasing after these popcorn bookies hass handed me this jump around junk.
[UK](con. WWI) F. Richards Old Soldiers Never Die (1964) 208: I told him it was a popcorn barrage compared to some I had been under.
[US]H. McCoy Kiss Tomorrow Goodbye in Four Novels (1983) 106: Jealous? Of him? That bum? That popcorn thief?
[US]T.R. Houser Central Sl. 41: popcorn [...] ‘get that popcorn mother fucker out of the house’.

2. (US) respectable, law-abiding.

[US]W. Burroughs Naked Lunch (1968) 159: Just look square, you dig, like a nice popcorn John.