loony adj.

late 13c., "affected with periodic insanity dependent on the changes of the moon," from Old French lunatique "insane," or directly from Late Latin lunaticus "moon-struck," from Latin luna "moon" (see luna).

Compare Old English monseoc "lunatic," literally "moon-sick;" Middle High German lune "humor, temper, mood, whim, fancy" (German Laune), from Latin luna. Compare also New Testament Greek selēniazomai "be epileptic," from selēnē "moon." Lunatic fringe (1913) was popularized and apparently coined by U.S. politician Theodore Roosevelt.

Then, among the wise and high-minded people who in self-respecting and genuine fashion strive earnestly for peace, there are foolish fanatics always to be found in such a movement and always discrediting it — the men who form the lunatic fringe in all reform movements. [Theodore Roosevelt, autobiography, 1913].

Earlier it was a term for a type of hairstyle worn over the forehead (1877). Lunatic soup (1918) was slang for "alcoholic drink" or several different alcoholic drinks drunk together.

mid-15c., lowen, louen "rascal, worthless person, boor," of uncertain origin, perhaps from Middle Dutch or Middle Low German; compare Dutch loen "stupid person" (16c.). The modern sense "crazy person" is by influence of loony.

large diving bird (especially the Great Northern Diver), 1630s, apparently an alteration of loom in this sense, which is from a Scandinavian source (compare Norwegian lom, from Old Norse lomr "loon").