Green’s Dictionary of Slang

to huddle together in a confined space; to associate with.

1900		Bulletin (Sydney) 25 Aug. 12/1: Why don’t yer let them other donahs see – / And all the Push – how much ye’re shook on me? / Why don’t yer mug and let the crawlers know – / Why don’t yer squeeze?
1918		‘Bartimeus’ Long Trick 85: ‘Mug up, and let’s get ’appy and chatty.’ They crowded together [...] for warmth.
1941		D. Burley N.Y. Amsterdam News 10 May 11: The hens [...] are just mugging (standing around).
1946		Mezzrow & Wolfe Really the Blues 94: A musician friend of mine [...] had to mugg with a lot of hophead gangsters who roomed near him.