belladonna (n.)

1590s, "deadly nightshade" (Atropa belladonna), from Italian, literally "fair lady" (see belle + Donna); the plant so called supposedly because women made cosmetic eye-drops from its juice (a mid-18c. explanation; atropic acid, found in the plant, has a well-known property of dilating the pupils) or because it was used to poison beautiful women (a mid-19c. explanation). Perhaps a folk etymology alteration; Gamillscheg suggests it is ultimately of Gaulish origin.