Things or people that growl, or a heavy glass jug used to transport draft beer (especially at breweries).
From 'growl' (possibly imitative in origin, to make a deep rolling sound) plus '-er' to indicate one who does the action. The beer jug meaning emerged in 19th-century American breweries where the metal lid allegedly made a growling sound.
The beer jug meaning is delightfully mysterious—no one's sure exactly why they're called growlers, but some historians think the lids made noise, others claim it's from the growling stomach of thirsty workers, or that they'd 'growl' about beer quality.
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