Green’s Dictionary of Slang

(US) a heavy drinking session.

1891		Salina Dly Republican (KS) 25 Sept. 3/2: Hummer — Drink big enough to make a policeman blush — referred to by bartenders as a ‘hog trough’.
1971		in Quarterly Journal of Studies on Alcohol XXXII 732: Hummer. Drinking bout [HDAS].
1972		Eble Campus Sl. Oct. 3: hummer – a beer or a drink.