Green’s Dictionary of Slang

1. the occupation by a drug dealer of several floors in a building; when one is raided others are still available.

1989

(con. 1982–6) T. Williams Cocaine Kids (1990) 53: Dealers either rent several apartments on a floor cheaply, or ‘squat’ illegally without paying any rent, usually working upward, a practice called ‘piggybacking’. Max explains: ‘The first place you wanna set up is the first floor, or the basement. Then you wanna take a second floor apartment and a third or fourth floor apartment, if you can. You move to other floors ’cause the first floor takes all the heat, and when it falls, you got other floors as backup.

2. the simultaneous injection of two drugs.

2001

ONDCP Street Terms 17: Piggybacking — Simultaneous injection of 2 drugs.