Green’s Dictionary of Slang

1. an outsize vial or rock of crack cocaine.

1988		in Spin Oct. 58: [...] selling ‘jums’ (or ‘jumbos,’ the big pieces of crack) [HDAS].
1995		L. Stavsky et al. A2Z 58/1: jumbs – n. crack-filled vials.
1998		Federal Criminal defense Update (Dist. Mass and NH) 14 Jan. 5/2: The defendant may not have had actual knowledge of ‘jums’ of crack found in search [etc.].
2002		at www.whitehousedrugpolicy.gov 🌐 What are crack cocaine price levels in the 20 Pulse Check cities? [...] Boston, MAL ‘jum’ (small rock) 0.1 gm $10/Double arrow.

2. a sealed plastic bag containing crack cocaine.

1996		US v. Andrade at caselaw.lp.findlaw.com 🌐 That bag, like the others, contained cocaine base (‘crack’); the two bags connected to Louis Andrade totalled some 14.21 grams in weight. That amount of cocaine is enough for 140 ‘jums’ (doses) or ‘dime bags.’.
2001		ONDCP Street Terms 13: Jum — Sealed plastic bag containing crack.
2005		Def Squad ‘Can You Dig It?’ 🎵 Order it now, no refunds / I’m like a clib with jums / I move crack fiends with different vowels.