Green’s Dictionary of Slang

B.C. adv.

[abbr.; crack n.7 + pun on chronological notation BC, before Christ]

(US black) Before Crack, i.e. many years ago.

[US]L. Stavsky et al. A2Z 6/1: B.C. – before crack: Six years B.C. their tape was goin’ round the Bronx.