cuckoo (n.) Look up cuckoo at Dictionary.com
mid-13c., 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 sense of "crazy" (adj.) is American English, 1918, but noun meaning "stupid person" is first recorded 1580s, perhaps from the bird's unvarying, oft-repeated call. The Old English name was geac, cognate with Old Norse gaukr, source of Scottish and northern English gowk. The Germanic words presumably originally were echoic, too, but had drifted in form. Cuckoo clock is from 1789.