/ luːn; NAmE / noun
1.
a large N American bird that eats fish and has a cry like a laugh
2.
= loony
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WORD ORIGIN
sense 2 late 19th cent.: from loon (referring to the bird's actions when escaping from danger), perhaps influenced by loony .
sense 1 mid 17th cent.: probably by alteration of Shetland dialect loom , denoting especially a guillemot or a diver, from Old Norse .