To drink something quickly without stopping, or the sound a train makes; also to move in a slow, steady way with repeated sounds.
Probably imitative in origin, imitating the sound of a train engine or repeated gulping; first appeared in English in the 1800s as railroad terminology.
The verb 'to chug' comes from onomatopoeia—the word literally sounds like a train chugging, which is why the same word means both the train sound and drinking fast—both involve repeated sounds in rhythm.
Complete word intelligence in one call. Free tier — 50 lookups/day.