I. cuck ‧ oo 1 /ˈkʊkuː $ ˈkuːkuː, ˈkʊ-/ BrE AmE 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’↑ nest s and that makes a sound that sounds like its name
II. cuckoo 2 BrE AmE adjective [not before noun] informal
crazy or silly:
You’re completely cuckoo!