But what is it a vestige of?

The Coccyx, funnily so called Kissy, is the vestige of our tail.

I've said before that I am very attached to mine and enjoy the choice of retracting (as in my recent pic in post a pic of your cock here) or letting it hang.

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Vigil, if you were my husband, I would give you poison.

Madam, if you were my wife, I would take it.