To spread a substance messily over a surface; a mark or stain made by smearing; an attempt to damage someone's reputation.
From Old English 'smerian' meaning 'to anoint with fat', related to 'smeoru' (grease). The figurative sense of damaging reputation developed in the 20th century from the literal sense of making something dirty.
The medical term 'Pap smear' uses the literal meaning of spreading cells on a slide, but the word's journey from describing greasy substances to character assassination shows how physical actions become metaphors for social behaviors. Politicians particularly dread 'smear campaigns' that can stick like the original greasy meaning suggests.
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