Green’s Dictionary of Slang

basing n.

[base n.1 (3)]

(drugs) using crack cocaine.

[US](con. 1982–6) T. Williams Cocaine Kids (1990) 135: basing smoking cocaine base; ‘dry’ (or ‘wet’) basing refer to absence (or presence) of liquid in the pipe.
[US]L. Stavsky et al. A2Z.