A phrase used to describe why you have a massive erection. This is mainly used to describe a bad habit of getting an erection whenever you see a female, even if they're not very attractive. The term comes from a comparison of a canine's tail "waggling" whenever it sees a bitch (a female dog). When a person asks about it, you simply say this phrase, and they will understand.
by The Tail Always Waggles March 15, 2009