Green’s Dictionary of Slang

mug n.3

(US black) ‘the act of putting on a shadow dance’ (Durst, The Jives of Dr Hepcat, 1953); also as v.

[US]Cab Calloway ‘Jitter Bug’ [lyrics] Toot your whistle and ring your bell, / Oh, butchie-wutchie, time will tell, / Don’t you worry, you just mug, / You’ll always be a jitter bug!
[US]L. Durst Jives of Dr. Hepcat (1989) 1: A group of fellows see a particular good dancer and are discussing him [...] ‘You better believe that he can work his ground smashers overtime. When that kitty puts on a mug it’s a killer.’.