A gray and black bird known for laying its eggs in other birds' nests, or slang for someone who is crazy or silly.
From Middle English 'cuckowe,' an imitative word echoing the bird's distinctive two-note call 'cuck-oo.' The word appears in multiple European languages with similar sound patterns. The slang meaning developed because cuckoos' parasitic breeding behavior seemed bizarre to people, eventually meaning 'crazy.'
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