Green’s Dictionary of Slang

v.b.c. phr.

[abbr. visible butt crack]

(US campus) having the outline of one’s buttocks showing through tight trousers; or revealing the top of one’s buttocks due to wearing one’s trousers lower than the waist.

[US]P. Munro Sl. U. 105: That guy walking in front of us had V.B.C.