Cavolo! You are out of hearts.
I was wondering was this meant so I looked it up and all I could find was "cabbage" and similar things. Now I'm curious as to why the owl says "cabbage" when you run out of hearts. Does it mean something else?
http://www.wordreference.com/iten/cavolo translates "cavolo!", rather charmingly, as "holy mackerel!". Basically, a mild expression of annoyance, like "darn it!". I've never actually heard an Italian say "cavolo"... unfortunately I can't tell you what they do say, because I think those words aren't allowed on Duolingo :)