Have sex a tergo, behind. It seems that the origin of the expression is in the 15th century, when Philip the fair, the womanizing husband of Juana la Loca, went up to the tower with one of palace ladies who used to be basin to see far cities of the Kingdom. The Queen told him: go the tower with this lady to wear it facing basin. The guards knew to what went and extended the expression with that sense.
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