Loon
late 19th century: from loon2 (referring to the bird's actions when escaping from danger), perhaps influenced by loony.
wiktionary
From Middle English louen, lowen(“rascal; rogue”), probably of Middle Dutch or Middle Low German origin. Compare Dutch loen(“simpleton”). Or, related to sense 2, due to the bird's loud cry. [1] Folk etymology associates it slang-wise with lunatic, though the latter may have influenced it; see loony.
Of North Germanic origin, from Old Norse lómr(“loon”), ultimately imitative of the bird's cry, particularly when it's in danger. Distantly related to lament, probably sharing Proto-Indo-European *leh₂- (expressive root).
etymonline
loon (n.1)
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").
loon (n.2)
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.
