Green’s Dictionary of Slang

1. (US Und.) a hoodlum, a thug, a gangster.

1975		E. Torres Carlito’s Way 44: I heard of spitters going down to Honduras [...] to ice a rat.
1977		E. Torres Q&A 125: Do we need these three spitters around, Mr. Petrone?

2. (Ulster) a form of home-made or converted gun.

2009

L. McIlvanney All the Colours 318: [S]porting shotguns with doctored stocks; converted starting pistols; ‘spitters’ knocked together by night-shift workers at light engineering firms.