Green’s Dictionary of Slang

1. (US) an act of masturbation.

1972

D. Jenkins Semi-Tough 170: They had a beat-off contest.

2. (US campus) an unpleasant person.

1975

G. Underwood ‘Razorback Sl.’ in AS L:1/2 56: beat-offn Unattractive, repulsive person.