It's not even an insult. I think I said it at least 3 times since this morning to each member of my family

Actually, I say it to myself whenever I do something silly.
It more or less has the same weight "Fuck off" has in the US or England.
I don't think Ljajic said that. He probably said something worse than that but Rossi didn't understand what was said exactly. He understood a word or two and thought it was the well known insult. And he can lie as much as he wants, but it was the sarcastic "well done maestro" which provoked the reaction.