What was Big Willy Shakes going for?
To get what's happening, we should all feast on a little history snack about the Elizabethan attitude toward money lending. The Church believed that interest should never be charged when one Christian loaned money to another Christian. The idea comes from Deuteronomy 23:19-23: "You shall not lend upon interest to your brother, interest on money, interest on victuals, interest on anything that is lent for interest. To a foreigner, you may lend upon interest, but to your brother, you shall not lend upon interest." Christians were allowed, however, to borrow money (with interest) from foreigners. Since Jews were classified as "foreigners" in England, they were encouraged to set up banks when they arrived.
Money is a very big deal in this play. (Big surprise there, right? The plot revolves around a Venetian merchant who can't repay a loan to a hated moneylender.) In much of The Merchant of Venice, the characters' attitudes toward wealth, mercantilism, and usury (lending money with interest) function as a way to differentiate between Christians and Jews.
The Christians in the play are portrayed as generous and even careless with their fortunes. The money-grubbing Shylock, on the other hand, is accused of caring more for his ducats than human relationships. At the same time, it seems that Shakespeare calls these stereotypes into question.
Here, Shylock tells us that Christians would borrow from anyone—a dog, even—if it would get them the money. They're waiting around with "bated breath," or a sense of anticipation, in even asking him for the dough. Sure, they spit on him last week, but they come running when they need it, don't they? Shylock's busy singing the tune (which is his go-to jam) of "my life's unfair because I'm a Jew."
Now we're not saying he doesn't have a point. He's been trash-talked, spit upon, called a dog, and worse, and it seems like there's not a scene that goes by in which some character isn't hurling anti-Semitic insults at him. Shylock is pointing out how hypocritical it is to treat someone like dirt and then ask them for a favor. Would you ask someone to spot you a $20 if you'd just beat them up? No way.
In fact, if we look closer, this term "bated breath" is really getting at that idea, too. Bated comes from the idea that someone is lowering himself for someone else, or being hostile (source). So Antonio and Bassanio think they're doing Shylock a favor by even talking to the guy. Yet, they're the ones asking him for money. Plus, they're jerks to him. Shylock is really saying that they're asking him with force or hostility while whispering because they're pretending they're above him.
See how this whole hypocritical thing works?