What are you looking at? Fatti i cavoli tuoi! (Mind your own business!)
I'm kidding, of course... really you should take a closer look. This unassuming leafy green is actually very useful linguistically. When used an expletive, it's a way to express irritation without resorting to more offensive, crude terms. So, when something doesn't go your way, you can say Ma che cavolo! ("but what a cabbage' or 'what the heck!')
Like many other veggies, it is also used to insult one's intelligence, so if someone says Sei una testa di cavolo, know they've just called you a cabbage head, or a dumbie.
And if all this has nothing to do with anything as far you're concerned, then you can simply say C'entra come i cavoli a merenda or literally, this fits in like cabbage at snacktime.
Ma che..
il cavolo
cabbage