Green’s Dictionary of Slang

beat-off n.

[backform. f. beat off v. (1)]

1. (US) an act of masturbation.

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

2. (US campus) an unpleasant person.

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