Green’s Dictionary of Slang

(US) a blow.

1850		Manchester Spy (NH) 21 Sept. n.p.: Ned would [...] fling in awful diffs with them windin’ blades.
1877		R. Burdette Rise and Fall of the Mustache 207: The dog [...] misunderstood the motion and thought his master was going to hit him a diff with that hat.
1879		Brenham Wkly Banner 28 Nov. 3/5: He hit him a beautiful diff on the head.
1889		J.C. Harris Balaam and Master n.p.: He got a big scyar on de side er his neck now what somebody hit ’im a diff [DA].
1903		Arizona Sentinel (Yuma, AZ) 18 Nov. 3: As he was climbing over the boar hit him a Jim Jeffries ‘diff’ in the side.