name given to various chronic skin diseases, later in more restricted use, 1530s, probably from leprous + -y (4). First used in Coverdale Bible, where it renders Hebrew cara'ath, which apparently was a comprehensive term for skin diseases. Also known as Hansen's disease (1938) for Norwegian physician Gerhard Henrik Armauer Hansen (1841-1912) who in 1871 discovered the bacillus that causes it.
The Middle English name for the disease was leper (mid-13c.), from Old French liepre and Latin lepra (see leper). But as the sense of this shifted after late 14c. to mean "person with leprosy," English began coining new nouns for the disease: lepri, leprosity, lepruse all date from mid-15c. but are now obsolete. A place for their treatment is a leprosarium (1846) or leprosary (1869, from French).
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.).
1899 in reference to U.S. foreign policy, "one who advocates a policy of non-participation in foreign affairs" (earlier in reference to treatment of leprosy), from isolation + -ist. As an adjective from 1920. Isolationism is attested in a general sense by 1902; in a U.S. geopolitical sense by 1919 in reference to opposition to joining the League of Nations.
1881, "a bleaching agent;" 1882, "an act of bleaching;" probably directly from bleach (v.). The Old English noun blæce meant "leprosy;" Late Old English also had blæco "paleness," and Middle English had blech "whitening or bleaching agent," but the modern words seem to be independent late 19c. formations from the verb.
infectious disease causing eruptions of rose-colored papulae, early 14c., plural of Middle English masel "little spot,"which isperhaps from Middle Dutch masel "blemish" (in plural "measles") or Middle Low German masele, both from Proto-Germanic *mas- "spot, blemish" (source also of Old High German masla "blood-blister," German Masern "measles").
There might have been an Old English cognate, but if so it has not been recorded. "The phonetic development is irregular" [OED] and the form might have been influenced by Middle English mēsel "leprous; a leper; leprosy" (late 13c., obsolete from mid-16c.), which is from Old French mesel and directly from Medieval Latin misellus "a wretch," noun use of an adjective meaning "wretched," a diminutive of Latin miser "unhappy, wretched, pitiable, in distress."