loony

(noun) someone deranged and possibly dangerous. Synonyms: crazy, looney, nutcase, weirdo.

(adj) informal or slang terms for mentally irregular. Synonyms: balmy, barmy, bats, batty, bonkers, buggy, cracked, crackers, daft, dotty, fruity, haywire, kooky, kookie, loco, loopy, nuts, nutty, round the bend, around the bend, wacky, whacky. Examples: "It used to drive my husband balmy."