Green’s Dictionary of Slang

(US Und.) a prohibitionist; a Prohibition agent.

1911		M. Monahan Test-bk of true Temperance 231: The prohis are constantly appealing to the people that all they ask for is that the people shall pass upon this question.
1921		Don Marquis Old Soak 85: Your feet, the Prohis say, go down to Hell [...] You made a Bar Fly of the Howling Swell.
1924		D. Hammett ‘Afraid of a Gun’ in Nightmare Town (2001) 228: The Prohis showed up there this morning and grabbed Lucky.
1931		Ade Old-Time Saloon 136: These ardent Prohis cannot think of any kind of drinking at any time except in terms of the saloon.
1949		Monteleone Criminal Sl. (rev. edn).
1950		Goldin et al. DAUL 164/1: Prohy. (During the Prohibition era) A prohibition agent of the pepartment of Justice or Treasury Department.