The story
In allusion to the Biblical character Esau, who sold his birthright for a meal of lentil stew (Genesis 25:29-34).
Etymology from Wiktionary, CC BY-SA 4.0.
sell one's birthright for a mess of pottage
Today it means: To make an unfavorable exchange, especially of something of great, but deferred value, for something of very low, but immediate value. Full definition →