Green’s Dictionary of Slang

mug v.2

also mugg
[? dial. muggle, a mess, a confusion, a disorder]

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

[Aus]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?
[UK]‘Bartimeus’ Long Trick 85: ‘Mug up, and let’s get ’appy and chatty.’ They crowded together [...] for warmth.
[US]Mezzrow & Wolfe Really the Blues 94: A musician friend of mine [...] had to mugg with a lot of hophead gangsters who roomed near him.