screwball
screw-ball
adjective
Eccentric or unconventional.
"That's a rather screwball idea, isn't it?"
noun
An eccentric or unconventional person.
"He's a bit of a screwball, but harmless."
Thesaurus
Synonyms: nut, crackpot, fruitcake, crank, nut case, half-baked, impractical, softheaded, crazy