loony
loo-ny
noun
A person considered to be mentally unstable or extremely foolish.
"Some old loony was shouting."
adjective
Mentally unstable or extremely foolish; mad.
"That's a completely loony idea."
Thesaurus
Synonyms: fruity, kooky, balmy, nuts, buggy, loopy, bats, whacky, wacky, barmy, around the bend, loco