Look at this. Wordreference.com is a very good online dictionary site.carpaccio - Dizionario Inglese-Italiano WordReference.com
Apart from that, there are plenty of words you won't find in many dictionaries. That doesn;t mean they're not part of the language. New words pop up all the time and are incorporated. And structurally, with the peggiorative ending "accio" it has all the signs of being an Italian word.
I've never seen it with mayonnaise, but from what I gather from what seem to be reliable italian sites, that's an ingredient. Here in Rome it's usually oil and parmigiano and then anything else that the chef fancies, including raw artichokes sliced thin. Mustard would be very unlikely, i'd think, but you never know, and I guess the main reason soemthing is called carpaccio is that it's thinly sliced and uncooked, and can be meat or fish or squash or artichokes or whatever the cook decides. But i've never been to Harry's bar so i have no idea what the original is like and even so, no law says people can;t take a good idea and fly with it.
a search on google in italian got me that it was invented by request of a regular at Harry's bar (owned by Cipriani, an italian, sponsored by a Harry, who was an American guy Cipriani lent money to and who came back years later and financed the establishment of the bar (as someone i think already mentioned).The "regular" was the countess Amalia Nani Mocenigo, who apparently was told by her doctor (in the 1950s) not to eat cooked meat and they say the name Carpaccio was in honor of the Venetian painter who used a particular brilliant red paint. (this, if true, would point to the beef theory of carpaccio, because veal would not be red.
But then, as i say, internet can be deceptive, we never really know who's writing the posts and if you skim through them they are full of incredibly contradictory information. The information about Harry and Cipriani is from the official site of Harry';s bar and should be reliable. Many other Italian sites describe the dish as being beef and mayonnaise and the story of the countess who couldn;t eat cooked meat.
Finally, you can see it's the weekend and I really don't feel like getting down to the work i have to do so I'm spending WAY too much time on internet!