Cornuto is, perhaps, the worst insult you can possibly give an Italian man. It is also pretty much the worst thing you can ever say about an Italian woman - provided you say it to her male partner.
It means, of course, that you are 'horned'. This is a metaphor that is largely lost to most of us English-speaking folks. If your wife has slept with other men, you grow little horns on your forehead. Who knows why. The insult also comes with a hand gesture. It's the same as the pointer-finger-and-little-finger-up-middle-fingers-held-by-thumb sign familiar to any fan of metal, or anyone who's ever known a fan of metal, or anyone who's ever seen a fan of metal walking down the street. Just like that, but with the 'horns' pointing towards your intended target.
It basically comes down to machismo, that uniquely Italian concept of masculinity. Machismo says that what makes you a man is what's in your pants. Everything about you should be focused on that and that alone. To be interested in anything else just shows what a total pussy you are.
When you curse someone with cornuto, you are saying that their woman sleeps with other men. Clearly, if they were not completely deficient in the bedroom, this would not occur. While you're at it, you're calling their girlfriend (fiancée, wife, whatever) a sexually liberated, unconcerned type. Obviously, this is all very serious business indeed, and I don't recommend any strangieri visiting Italy start tossing this insult around - you'll probably end up bleeding to death in an alley somewhere while some Italian guy uses your passport to traffic drugs.
The funniest thing I ever saw in Italy was an argument between two men involved in a minor accident. The car in front had to suddenly stop and the one behind gently bumped its rear bumper.
I should probably point out at this point that arguing loudly with total strangers is the most popular national sport after soccer. They will say the most horrifyingly cruel things to someone who digresses from the cast-iron rules of Italian society - such as, say, looking before crossing the street, or wearing any item of clothing worth less than the GDP of assorted Pacific and Caribbean countries. In short, they are like French people, only in tighter pants, and they usually manage to spit less when they talk.
So, the driver of the first car flings open the door of his car (a Citroën, if I remember correctly. Only Europeans would actually name a brand of cars 'lemons'), shouting obscenities at the driver of the second car, waving his arms wildly, with small drops of spittle flying out of his mouth. My Italian at the time was not quite good enough that I could make out actual words, but really, I don't think anybody there could.
He stomps up to the other driver, who by now had pulled himself from the mangled wreckage of his car, and yells at him some more. Faced with the frenzied indifference of his target, he slowly turns red as his fury increases.
Following is a transcript, with translations, of what ensued. (Note: This is from memory, and my Italian's pretty terrible. If my Italian is wrong, you know what to do)
First man: "Cornuto su te!"
"Sir, it seems to me you are a cuckold."
Second man: ...!
(Clutching at chest as if stabbed to his very core)
First man: "Cornuto sulla sua padre!"
"In fact, from the look of you I suspect your father was a cuckold as well. From this one can also deduce that your mother had questionable morals, and you are probably an illegitimate child."
Second man: ...!
(Trying to dodge the thin strings of mucus flying at his face)
First man: "Cornuto sulla padre dalla sua padre!"
"Furthermore, on review of your recent behaviour, it strikes me that your father's father was a cuckold, and your grandmother really not the marrying type, as they say."
Second man: ...!
(Shocked into a dazed, coma-like state, he hereby withdraws from the conversation.)
First man: "Cornuto sulla padre dalla padre dalla sua padre!"
"I would further conject that your father's father's father was a cuckold. Clearly, this ill stretches far back in your lineage, and you yourself are so tainted with its evil that this could explain the astonishing depth of your sociopathy."
First man: "Anche, cornuto sul suo figlio!"
"My anger at the manner of our meeting is such that I wish cornuto upon your son. Considering your family history, this shouldn't come as any surprise to you."
First man: "E sul figlio dal suo figlio!"
"Your grandchildren, also, shall suffer the consequences of your abhorrent behaviour today."
First man: "E sul figlio dal figlio dal suo figlio!"
"Needless to say, you grandson's children cannot escape the curse that I hereby cast upon the many generations of your family that have contributed to my cost and inconvenience this dark day."
Having soothed his rage, this upstanding member of society then calmly adjusted his Gucci suit, flicked a spittle orphan off his sleeve, returned to his avenged car, and placidly drove away.
The second man was rushed to hospital suffering severe Cornuto trauma.
The Debutante points out re Cornuto: The English translation is, of course, cuckold. Of course. But thank you, Deb!