so
/ˈso/ · B1 · adv
so
so tonto = you utter fool; so is a worn-down señor, so it means Mr. Fool
Used only before an insult: ¡so idiota!, ¡so bruto! It comes from señor, worn down through seor to so, so the insult is a mock title, like Mr. Idiot. It intensifies the insult, much like English bloody or blithering.