Drinking something quickly in large gulps, or the sound an engine makes when running inefficiently.
From the onomatopoeia 'chug,' imitating the sound of an engine; the drinking meaning applied the sound metaphorically to drinking beverages noisily.
The word 'chug' is onomatopoeia—it literally sounds like what it means, whether an engine or someone drinking—which is why it's the same sound in dozens of languages.
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