droit du seigneur (n.)
1784, alleged medieval custom whereby the feudal lord had the right to have sex with the bride of his vassal on their wedding night before she went to her husband, from French, literally "the lord's right." There is little evidence that it actually existed; it seems to have been invented in imagination 16c. or 17c. The Latin form was jus primae noctis, "law of the first night." For French droit, see right (adj.2).