Here's the answer, I've been wondering about.
According to Wama Underwear, the reason it's called a PAIR of underwear when it's just ONE - The term pair of underwear, or pair of pants, or pair of knickers, what have you, comes from the fact that early underwear, worn in medieval times, was two pieces. Yep, you'd pull a piece up each leg, and then tie them together! And that's your pair.