In the beginning of Parashat Vayechi, Yaakov asks Yosef to place his hand under his thigh and promise not to bury him in Mitzrayim. Rashi explains that this action was the sign of a Shevuah, an oath. The Maharal asks, why does Yaakov require Yosef to make the Shevuah in this way? Why couldn’t Yosef just swear by receiving an object from Yaakov like the protocol in other Shevuot? It is understandable regarding Avraham Avinu why he made Eliezer swear by placing his hand under Avraham’s thigh, namely, where his Brit Milah took place. As Rashi elucidates, this Mitzvah of Milah was the first given directly to Avraham and it came through much pain and was therefore precious to him. This idea, however, would not make sense concerning Yaakov unless we said that he just copied his grandfather, Avraham. Also, Yosef makes a Shevuah with his brothers but there is no mention of a Mila. So why does Yaakov make his Shevuah by employing this method of swearing over a thigh?
The Maharal answers that this was the way the people back then made Shevu’ot; the one swearing would place their hands under the other thigh of the person he is swearing to (as the Ibn Ezra points out in his commentary to Breishit 24:2 and confirmed by Da’at Mikra ad. loc.). Yaakov thought that if he did not do the Shevuah this way, when Yosef would ask him to go and bury his father, Paroh would respond that he did not make the Shevuah according to the law of the land, and, therefore, the Shevuah was null and void. Yaakov’s Shevuah to Yosef was made so Paroh would allow Yosef to bury him in Eretz Yisrael. However, one could say that Paroh still may not honor this Shavuah. Rashi answers that Yosef swore to Paroh that he would not let the people know that Paroh could not speak Hebrew, thereby ruining his godlike, all-knowing image. This Shavuah was what ensured that Paroh would honor any legitimate oath made by Yosef, as is if he did not Yosef could justifiably break his oath. This explains why Yaakov made Yosef swear over his thigh.