Green’s Dictionary of Slang

chug v.1

[abbr. chug-a-lug v.]
[1950s+]

1. to drink down in a single draught, to drink quickly; thus chugger n., a drinker; (US campus) chugging contest n., a drinking competition in which each contestant has to down a succession of drinks in a single swallow.

2. in fig. use, ‘to swallow’.