tansy (n.)

perennial herb native to northern Eurasia, mid-13c., from Old French tanesie (13c., Modern French tanaisie), from Vulgar Latin *tanaceta (neuter plural mistaken for fem. singular), from Late Latin tanacetum "wormwood," from shortened form of Greek athanasia "immortality," from athanatos "immortal," from a- "not," privative prefix, + thanatos "death" (see thanatology). So called probably for its persistence. English folklore associates it with pregnancy, either as an aid to contraception or to provoke miscarriage.