It's a misspelling of the Italian interjection "mamma mia!", which literally
means "oh my mother!".
It is used by italians in quite tha same way as English
use the expression
"oh my God!", except ...Italians do NOT say "mamma mia!" when they're making love with their girlfriend!
"Hai visto quel film
horror?" "Mamma mia!! Era così spaventoso!" (= "Have you seen that horror movie?" "Oh my God!! It was so frightening!")
There's a musical
called "mamamia": is it about Italian mafia?