Nouns

(n)crackpot, crank, nut, nut case, fruitcake, screwballa whimsically eccentric person
(n)screwballa pitch with reverse spin that curves toward the side of the plate from which it was thrown

Adjectives

(s)crazy, half-baked, screwball, softheadedfoolish; totally unsound“a crazy scheme”, “half-baked ideas”, “a screwball proposal without a prayer of working”