1570s, "to corrupt, contaminate," also "to touch, tinge, imbue slightly" (1590s), from Middle English teynten "to convict, prove guilty" (late 14c.), which is partly from Old French ataint, past participle of ataindre "to touch upon, seize" (see attainder). It also is from Anglo-French teinter "to color, dye" (early 15c.), from Old French teint (12c.), past participle of teindre "to dye, color," from Latin tingere (see tincture). Related: Tainted; tainting.
c. 1600, "stain, spot," from Old French teint "color, hue, dye, stain," from Latin tinctus "a dyeing," from tingere "to dye" (see tincture). Meaning "a moral stain, corruption, contaminating influence" is from 1610s.
also 'taint, representing a colloquial contraction of it ain't, by 1830s, American English. The joke about the perineum involving this word dates to at least 1977.