nitwit
dickhead (slang) dickwit (slang) dimwit (informal) dipstick (Brit. slang) divvy (slang) dork (slang) dummy (slang) fool, fuckwit (taboo slang) geek (slang) gobshite (Irish taboo slang) halfwit, lamebrain (informal) nincompoop, ninny, numpty (Scot. informal) oaf, plank (Brit. slang) simpleton