The word “mad” derives from the Latin, mūtāre, to change. Originally “mad” was used to describe a dog whose behavior had changed because it had contracted rabies. By Shakespeare's time, “mad” was applied to people in the same several senses which we use today. Bianca describes both her sister and Petruchio as mad. For Petruchio, she may mean insane, or lunatic. But for her sister, and quite possibly for Petruchio as well, she’s using the milder sense of unconventional, or outrageous.