cuckoo (n.)

European bird noted for its love-note cry and notorious for parasitism, c. 1300, cokkou (late 12c. as a surname), from Old French cocu "cuckoo," also "cuckold," echoic of the male bird's mating cry (compare Greek kokkyx, Latin cuculus, Middle Irish cuach, Sanskrit kokilas).

Slang adjectival sense of "crazy" is American English, 1918, but noun meaning "stupid person" is recorded by 1580s, perhaps from the bird's unvarying, oft-repeated call. The Old English name was ʒeac, cognate with Old Norse gaukr, source of Scottish and northern English gowk, which also has insulting senses. The Germanic words presumably originally were echoic, too, but had drifted in form. Cuckoo-clock is from 1789.

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Definitions of cuckoo from WordNet
cuckoo (n.)
a man who is a stupid incompetent fool;
Synonyms: fathead / goof / goofball / bozo / jackass / goose / twat / zany
cuckoo (n.)
any of numerous European and North American birds having pointed wings and a long tail;
cuckoo (v.)
repeat monotonously, like a cuckoo repeats his call;