Green’s Dictionary of Slang

(US) a job, employment, a difficult chore.

1902		S.E. White Blazed Trail 95: And now this condigned jobber ties us up for a million and a half .
1930–1		J.T. Farrell ‘Spring Evening’ in Fellow Countrymen (1937) 175: Well, if I didn’t have me a good jobber already, I’d take a chance at something like that.
1955		(con. 1944) H. Robbins A Stone for Danny Fisher 282: He was a salesman for one of the big cigarette-jobbers who specialized for the most part in big black-market deals.
1971		E.E. Landy Underground Dict. (1972) 113: jobber n. Difficult undertaking.