loon (n.1) Look up loon at Dictionary.com
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) Look up loon at Dictionary.com
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.