Green’s Dictionary of Slang

a blow to the head.

1824		Morn. Chron. (London) 21 Apr. 4/3: He was again met with an under hand jobber on the muzzle.
1827		Egan Anecdotes of the Turf, the Chase etc. 82: Hard nobbers, and jobbers, and claret galore.
1834		Ely’s Hawk & Buzzard (NY) Sept. 6 n.p.: Sam [...] delivered a heavy left hand jobber.