Green’s Dictionary of Slang

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

1893		F. Harris ‘Gulmore, the Boss’ in Elder Conklin & Other Stories (1895) 173: What he said was popcorn; but it took with the Mugwumps.
1899		W.J. Kountz Billy Baxter’s Letters 58: He is tight-ribbed and popcorn.
1907		News & Courier (Charleston, SC) 14 Apr. 18/1: Chasing after these popcorn bookies hass handed me this jump around junk.
1933		(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.
1948		H. McCoy Kiss Tomorrow Goodbye in Four Novels (1983) 106: Jealous? Of him? That bum? That popcorn thief?
1987		G.W. Clemente Cops are Robbers 50: The safes were, as Bucky put it, popcorn. [...] Bucky and Kenny opened them like sardine cans .
1985		T.R. Houser Central Sl. 41: popcorn [...] ‘get that popcorn mother fucker out of the house’.

2. (US) respectable, law-abiding.

1959

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