Genesis 25:33-34

GOD’S WORD Translation

³³ “First, swear an oath,” Jacob said. So Esau swore an oath to him and sold him his rights as firstborn. ³⁴ Then Jacob gave Esau a meal of bread and lentils. He ate and drank, and then he got up and left.

This is how Esau showed his contempt for his rights as firstborn.