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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.
Dude: "Woah! What's that in your pants?!"

Brandon: "I'm sorry... The tail always waggles."
by The Tail Always Waggles March 14, 2009
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