Green’s Dictionary of Slang

(drugs) a process that alters the colour of cocaine to light brown, thus making it appear purer than it actually is.

1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 135: beiging chemically altering the cocaine so that it looks brownish, for those who think this indicates purity.
2001		ONDCP Street Terms 3: Beiging — Chemicals altering cocaine to make it appear a higher purity; chemically altering cocaine to make it look brown.