A bawd is a pimp or a madam who procures clients for his or her prostitutes. Since the nurse is Juliet’s overseer and has come to see Romeo, Mercutio is jokingly pretending she’s procuring Romeo for Juliet (as it turns out, he’s not too far off the mark). But bawd is also another name for a hare, a rabbit, and “Soho” is the call that a hunter makes to alert the other hunters when he’s flushed out a hare.