Green’s Dictionary of Slang

1. (US) a long-barrelled revolver; thus long cut short, a sawn-off revolver [abbr.].

1933		N. Algren ‘So Help Me’ in Texas Stories (1995) 25: He says I can have the roscoe if I wish to be selfish, but he will get a long cut short for hisself.
1947		N. Algren Neon Wilderness (1986) 285: Fort [...] woudn’t quit fingerin’ the trigger of that long for a minute.

2. (US black) money [abbr. long green n.].

1993

J. Mowry Six Out Seven (1994) 433: Sebastian had often wondered where Hobbes stashed what had to be some major-time long.

3. (US) $1000 [long green n.].

1999		J. Ridley Love Is a Racket 111: To win the kind of dough I had to have, nearly thirty long [...] we needed to gamble outside one of the legal joints.
2005		J. Stahl I, Fatty 223: Violating the 18th Amendment is going to cost you another 50 long.