The joke comes to us from Jean de Conde of Hainaut, born 1275:
A game of truth-telling is being played at court by a Queen and her retinue. A knight is asked by the Queen if he has fathered any children; he is forced to admit that he has not.
The Queen nods in assent, saying "you do not have the look of a man who could please his mistress when you hold her naked in your arms.
For your beard is little more than the kind of fuzz that ladies have in certain places, and it is easy to tell from the state of the hay whether the pitchfork is any good."
On his turn, the knight asks "Lady, answer me without deceit.
Is there hair between your legs?" When she replies, "none at all", he comments, "Indeed I do believe you, for grass does not grow on a well-beaten path."