According to Pellegrino Artusi, the history of Arista coincides with the story of the Byzantine Patriarch Bessarion and his arrival in Florence in 1430 for the ecumenical council.
While tasting an excellent pork roast he said “Aristos!”, the greek word to express "best, excellent". The others sitting around the table thought that he was simply calling the roast pork by its greek name. This is how they started calling pork roast arista. Now, this little story is interesting, unfortunately it can't be true since a century before Bessarion's arrival in Tuscany people were already talking about arista.
Thus, we are left with two possible hypotheses: a) Arista is a greek word and in the 13th century there were greeks living in Florence (they were perfume merchants) and their presence is also testified by the neighborhood and street called "Borgo dei Greci". b) Arista comes from the latin arista that also means upper part, like the pork shank? Why not leave your opinion on this tasty and ancient query?