According to Shylock in The Merchant of Venice, how had Antonio insulted him in the past?
Antonio insults Shylock in several ways. He calls him a "cur" which is a dog, and he does it in public in front of other businessmen upon whom Shylock depends for his own livelihood. He curses Shylock's religion and ethnicity by mocking him and consistently mentioning what he sees as idiosyncrasies of Judaism. Most significantly, he spits on Shylock in front of others, demonstrating his disdain for the usurer.
Shylock seems to give the most weight to being called a "cur," most likely because of the dehumanization that such an insult suggests.