The sound made when liquid pours out of a bottle quickly, or to drink something in large gulps.
Imitative/onomatopoetic word—it literally imitates the sound of liquid pouring or being swallowed. First appeared in English writing in the 19th century, becoming more common in the 20th century.
The sound 'glug' changes based on physics—the pitch actually depends on how quickly air enters the bottle as liquid leaves, which is why pouring from different angles makes different sounds, and why that satisfied 'glug' is so satisfying to your ears.
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