Green’s Dictionary of Slang

short arm n.

also short-arm tail
[similar to third leg under third adj. but note G. Williams (1994) who makes a link to 17C milit. metaphor arms, the genitals]

the penis, usu. found in compounds below.

In compounds

short-arm heister (n.) (also short-arm bandit) [heister n.]

[1950s+] (US Und.) a rapist; thus short-arm heist, rape.

short-arm inspection (n.) (also short arm, short arm drill, short arm physical)

[1910s+] (orig. milit.) medical inspection of the genitals; thus short-arm inspect v.