What's the "een" in Halloween?
mid-13c., from Old French cocu "cuckoo," also "cuckold," echoic of the male bird's mating cry (cf. 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.
Crazy; very eccentric; nutty: Where do you get these cuckoo ideas? (1918+)noun
A crazy or eccentric person (1581+)
[perhaps because of the bird's monotonous, silly-sounding call]
(Heb. shahaph), from a root meaning "to be lean; slender." This bird is mentioned only in Lev. 11:16 and Deut. 14:15 (R.V., "seamew"). Some have interpreted the Hebrew word by "petrel" or "shearwater" (Puffinus cinereus), which is found on the coast of Syria; others think it denotes the "sea-gull" or "seamew." The common cuckoo (Cuculus canorus) feeds on reptiles and large insects. It is found in Asia and Africa as well as in Europe. It only passes the winter in Palestine. The Arabs suppose it to utter the cry _Yakub_, and hence they call it _tir el-Yakub_; i.e., "Jacob's bird."