Green’s Dictionary of Slang

(US milit.) the regimental prophylaxis clinic, issuing contraceptives and dealing with venereal disease.

1921		Dos Passos Three Soldiers 202: ‘That’s one thing you guys are lucky in, don’t have to worry about propho.’ The orderly wrinkled his face up and winked elaborately.
1925		Dos Passos Manhattan Transfer 281: Just my propho kit . . . Wont need it no more.
1928		A.C. Inman 14 Jan. diary in Aaron (1985) 354: So it was drink, screw and propho. I was lucky. I didn’t catch nothin’.
1932		(con. 1919) Dos Passos Nineteen Nineteen in USA (1966) 725: Wet French postcard dream began with saltpeter in the coffee and ended at the propho station.