ergot (n.)

fungal disease of rye and other grasses, 1680s, from French ergot "ergot," also "a spur, the extremity of a dead branch," from Old French argot "cock's spur" (12c.), which is of unknown origin. The blight so called from the shape the fungus forms on the diseased grain. Related: Ergotic. An alkaloid from the fungus, ergotamine (1921) is used to treat migraines.