Green’s Dictionary of Slang

1. (US) an act of masturbation, also attrib.

1972		D. Jenkins Semi-Tough 170: They had a beat-off contest.
2009		D.W. Gunn Golden Age of Gay Fiction 105/1: We were a fast, efficient operation, the Cadillac of the beat-off book business [Simes:DLSS].

2. (US campus) an unpleasant person.

1975

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