Deuteronomy 25:9

In his face, or presence, upon the ground, as appears from the Gemarra of Jerusalem, where we read this form: (Haydock) "In our presence, (the three judges are specified) N, widow of N, hath taken off the shoe of N, son of N. She brought him before us, and took off the shoe from his right foot, and spat in our presence, so that we saw her spittle upon the ground; and she said to him, So shall he be treated who will not establish the house of his brother." Before this ceremony took place, the widow was obliged to wait three months, to prove that she was not in a state of pregnancy; for if she were, the brother could not marry her. He was never obliged to do it, but if he refused he was deemed infamous. The taking off the shoe was intended to humble him, as well as to shew that he relinquished all his claim to the inheritance. Josephus ([Antiquities?] v. 11) says, that Ruth gave the relation, who would not marry her, a slap on the face, or rather, as it ought to be printed, "she spat in his face," which was a mark of the greatest ignominy, chap. xii. 14., Isaias l. 6., and Matthew xxvi. 67. (Calmet)