tansy (n.)

perennial herb native to northern Eurasia, used medicinally or in cooking, mid-13c., tansei, 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).

Said to be so called probably for its persistence. English folklore associates it with pregnancy, either as an aid to contraception or to provoke miscarriage. As "pudding flavored with the juice of tansy," mid-15c.