Sticky, slimy, or messy unidentified substance that's often dirty or unpleasant.
Possibly from Scandinavian 'gunk' or influenced by 'gunky' (sticky), first appearing in American English in the early 20th century as informal slang for unidentifiable matter.
The fact that we needed a whole casual word for 'unidentified sticky stuff' shows how modern industrial life created situations ancient people never had—old languages don't have good words for it.
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