The original text has "Brach," not "Breathe." However in the previous line the "Lord" told his huntsman to take good care of his hounds, and so it would make sense that he would go on in the same vein and tell the huntsman to give the hound Merriman a chance to catch its breath. On the other hand, "Brach"which means "bitch hound"makes no sense in this context. The Norton Shakespeare also substitutes "Breathe" for "Brach" here.