Green’s Dictionary of Slang

thoroughly, totally, keenly; superlatively.

1835		D. Crockett Col. Crockett’s Tour to North and Down East 192: He went jam up for war.
1941		‘Here & There’ in N.Y. Age 15 Feb. 10/6: What lad [...] is playing a chick upstate jam up but whose heart interest is locally?
2001		D. Jenkins Money-Whipped Steer-Job 65: I was playing jam-up, lights-out tee to green.
2008		D. Jenkins Franchise Babe 199: Ginger played jam-up, threw down a 71.