"one afflicted with leprosy," late 14c., earlier "the disease leprosy," from Late Latin lepra, from Greek lepra "leprosy," noun use of fem. of lepros (adj.) "scaly, scabby, rough, leprous," related to lepein "to peel," from lepos, lepis "a scale," from PIE root *lep- (1) "to peel," which also yields words for "something delicate and weak," via the notion of "small shaving, flake, scale" (cognates: Latin lepidus "pleasant, charming, fine, elegant, effeminate," lepos "pleasantness, agreeableness;" Old English læfer "rush, reed; metal plate;" Lithuanian lopas "patch, rag, cloth," lepus "soft, weak, effeminate").
Originally in Middle English this was the word for the disease itself (mid-13c., via Old French lepre); the shift in meaning to "person with leprosy" perhaps developed in Anglo-French, or is because the -er ending resembled an agent-noun affix. By mid-15c. other nouns for the disease were being coined (see leprosy). In English lepra also was an old name for psoriasis (late 14c.).