Green’s Dictionary of Slang

(orig. US prison) to attack in a group.

1967		Wenkert et al. Racial Crisis in Richmond, CA 81: Most versions of the story were that Crowder had been ‘ratpacked’ and beaten by a gang of 3.
1975		J. Carr Bad (1995) 47: Individuals or weak groups walking alone got ratpacked.
1988		D. Woodrell Muscle for the Wing 95: A group of neighborhood lads [...] prowling for ethnic winos or solvent strangers to ratpack.
1992		Bentley & Corbett Prison Sl. 90: Rat Packed [...] When three or more inmates attack one [...] generally associated with established gang activity.
1993		K. Scott Monster (1994) 149: Everyone wanted to rat-pack him, but Monk insisted on it being head up.
2001		(con. 1975–6) E. Little Steel Toes 21: Kicking one of the negroes that are rat-packing Red in the middle of the back.