To damage the purity or reputation of something by contaminating it with something bad.
From Old French 'teinte' meaning dye or color, derived from Latin 'tinctus' (past participle of tingere, to dye). The meaning shifted from a physical stain to a metaphorical mark of dishonor or corruption.
The word 'taint' originally just meant any color or dye, but people realized that staining something also spoils its reputation—just like a scandal can 'stain' a person's entire career even if it's just one mistake.
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