insaniac /ɪnˈseɪniˌæk/adjective 1noun 1forms 3 adjective1 •(slang) Insane and maniacal.And although his insaniac towing of Jezebel had demonstrated that he might be just as crazy as the rest of them, it was hard to sit in this new Bronco and feel bad anymore about their old car. — 2014 noun1 •(slang, offensive) One who is insane and maniacal. formsinsaniacs · more insaniac · most insaniac