Porco Dio (lit. "pork God", where "pork" is an insult), also said and spelled as "porcodio" or "porcoddio" (every form has slight differencies of using) is an italian expression part of the huge group of italian cuss words known as "bestemmie". It's the most used and basic bestemmia and like all the other ones it's highly offensive and should only be used among friends who you know would not get offended. It's used basically in every kind of situation but it's real purpose is to be used mixed with some good italian hand gestures as the ultimate imprecation when you are REALLY mad. Don't over use it tho because only 12 yo kids think it's cool