Green’s Dictionary of Slang

1. a gun, a pistol.

1882

T.C. Harbaugh Bravo Bill 2: I’ll trouble you by askin’ you to lower yer dropper [HDAS].

2. a paid killer.

1926		J.J. Finerty Criminalese.
1931		‘Und. “Lingo” Brought Up-to-Date’ L.A. Times 8 Nov. K3: DROPPER: Professional killer; machine gunner: a sure-shot gunman.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 62/2: Dropper. A professional assassin.
1960, 1975		Wentworth & Flexner DAS.
1962		Ragen & Finston World’s Toughest Prison 797: dropper – A paid killer.

3. (US prison) one who carries a knife.

1934

J.L. Kuethe ‘Prison Parlance’ in AS IX:1 26: dropper. A knife man.

4. (US Und.) a violent robber.

1931		G. Milburn ‘Convicts’ Jargon’ in AS VI:6 438: dropper, n. A thief that knocks out his victims; this is considered an art.
1960, 1975		Wentworth & Flexner DAS 163/2: dropper n. Any of various criminals who ‘drop’ their victims; specif., a thief who knocks out or stabs the person he robs.