What does Genesis 25:33 mean?

Esau, exhausted from working in the field, has requested some of Jacob's red stew. Jacob has demanded Esau's birthright as payment (Genesis 25:29–31). Esau has foolishly and hastily agreed. Now Jacob closes the deal by asking Esau to bind himself with an oath. Oaths were taken with great seriousness in this era. Esau quickly swears the oath and sells his firstborn rights.

We're amazed at both men here. First, Esau strikes us as a careless fool who would value his birthright so little. He might have thought that Jacob was joking, at first. But asking him to swear an oath, in that day, was the equivalent to signing a modern written contract. Esau is, at the very least, being reckless.

Second, Jacob comes off as cold and calculating, unconcerned about taking advantage of his brother's irresponsibility and need for immediate gratification of his appetite. He's leveraging the situation for his own benefit, and at great cost to Esau. Neither brother is very likable in this moment.