Green’s Dictionary of Slang

trigger n.²

the penis.

1832		Satirist (London) 16 Sept. 301/4: O, John, thou art my chosen trigger-man! / [...] / Go Peter, go, thou’rt past thy vigour, man. / I might as well embrace a nigger-man.
1939		J. Steinbeck Grapes of Wrath (1951) 12: We spied a nigger, with a trigger that was bigger than an elephant’s proboscis or the whanger of a whale.
1941		in G. Legman Limerick (1953) 52: A young man maintained that his trigger / Was so big that there weren’t any bigger.