Green’s Dictionary of Slang

beat off v.

[SE beat/beat one’s meat under meat n. + come off under come v.1 ]
[1960s+] (US)

1. (also beat) to masturbate; usu. but not exclusively of men.

2. to masturbate another person.

3. in fig. use, to waste time, to loaf around.

In compounds

beatmeat (n.) [meat n. (2)]

[1980s] (US gay) masturbation; the post-ejaculatory penis.

In phrases

have a beat (v.) [1980s] (Aus.)

to waste time, to mess about.

take a beating (v.) [fig. use of SE]

[1960s+] to masturbate.