cuckoo
adj.adj. silly or crazy in a way that is hard to believe. You use this word when someone is acting very strange or has a wild idea.
adj. mentally unbalanced or foolish. Often used predicatively after linking verbs like 'go' or 'be' to describe irrational behavior.
That plan sounds completely cuckoo to me.
The neighbors thought he had gone cuckoo when they saw him mowing his lawn at three in the morning.
While the proposal was initially dismissed as cuckoo by the board, it eventually proved to be the only viable solution to the company's mounting debt crisis.
From Middle English cokkou, kokkow, cukkuk, gokkouȝ, probably from Old French cocu, coquu, cucu (whence French coucou); ultimately onomatopoeic of the song of the male Common Cuckoo (Cuculus canorus), perhaps via Latin cucūlus (“cuckoo”). Compare dialectal English gowkoo (“cuckoo”). Displaced Middle English gnokken (“cuckoo”) and native Middle English yeke, ȝek (from Old English ġēac (“cuckoo”)), see English gowk. The UK sense is by analogy from the bird's practice of brood parasitism.
Informal; typically follows a linking verb like 'be', 'go', or 'become'.