John Wesley on Numbers 5:21

An oath - That is, a form of cursing, that when they would curse a person, they may wish that they may be as miserable as thou wast. Thy thigh - A modest expression, used both in scripture, as Gen 46:26, Exo 1:5, and other authors. To rot - Heb. to fall, that is, to die or waste away. To swell - Suddenly and violently till it burst, which the Jews note was frequent in this case. And it was a clear evidence of the truth of their religion.