bug, crackbrain, crazy, fool, fruitcake, head case, loon, loony, lunatic, maniac, nut, nutcase, nutter(British slang), psycho, psychopath, sicko, wacko(also whacko)

madman, madwomanabnormal, deviantmanic-depressive, monomaniac, mythomaniac, neurotic, obsessive, obsessive-compulsive, paranoid, psychoneurotic, psychotic, schizo, schizoid, schizophrene, schizophrenic, sociopathcharacter, codger, crack, crackpot, crank, eccentric, fantastic, flake, kook, oddball, oddity, original, quiz, screwball, tyke(also tike, chiefly British), weirdo, zanycase, patient