cuckoo
[noun]
Synonyms: crackpot, crackbrain, crank, ding-a-ling, harebrain, kook, lunatic, nut, screwball
[adjective]
Synonyms: insane, crackbrained, cracked, crackers, cracky, cranky, crazed, crazy, daffy, daft

[TahlilGaran] English Synonym Dictionary ▲

I. cuck‧oo¹ /ˈkʊkuː $ ˈkuːkuː, ˈkʊ-/ noun [countable]
[Date: 1200-1300; Language: Old French; Origin: cucu, from the sound it makes]
a grey European bird that puts its eggs in other birds’nests and that makes a sound that sounds like its name

[TahlilGaran] Dictionary of Contemporary English ▲

II. cuckoo² adjective [not before noun] informal
crazy or silly:
You’re completely cuckoo!

[TahlilGaran] Dictionary of Contemporary English ▲