To damage someone's reputation by spreading negative information or accusations, or to apply a black coating or finish.
From 'black' + 'wash,' a 20th-century term formed by analogy with 'whitewash,' meaning to cover something dark or damaging (reputation) versus whitewashing which covers up wrongdoing.
The term 'blackwash' is rarely used compared to its opposite 'whitewash'—linguists note this interesting asymmetry where the white metaphor dominates English idioms about reputation and truth!
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