To spread something wet, sticky, or dirty across a surface with a rubbing motion; or to damage someone's reputation with accusations.
From Old English 'smeoru' (fat, grease), the word originally described physically spreading greasy substances. By the 17th century, it also developed the figurative meaning of damaging someone's character.
The double meaning of 'smear' is fascinating—a smear campaign in politics literally tries to 'dirty' someone's reputation the same way you'd smear grease on a surface.
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