Green’s Dictionary of Slang

(orig. US black) exciting, pleasing, excellent, the best; also as an excl. of pleasure.

1942		Pittsburgh Courier (PA) 11 Apr. 20/2: The Onyx Club [...] will try it again, opening next week with a jammin’ colored review.
1982		Eble Campus Sl. Spring 5: jamming – having qualities of excellence.
1988		Eble Campus Sl. Mar. 6: jammin’ [...] expression of elation: ‘Hey, no class tomorrow!’ ‘Jammin’!’.
1993		R. Shell Iced 113: There were lots of jammin’ events at different schools that I wanted to go to.
2000		W.T. Vollmann Royal Family 534: I’ve got two projects [...] and Sunday I’ve got a corporate brunch, she said. Today I was really jammin’, like they say.