The adash is translated as the ‘lentil’ in the NIV. The lentil (Lens culinaris) is a bushy annual plant from the legume family. It produces an edible seed for both human and animal consumption.

Lentils

Different types of lentils. PICTURE: Justinc/Wikipedia (licensed under CC BY-SA 2.5)

The adash is mentioned in a list of produce that was given to David and his entourage when he was fleeing Absalom and his supporters.

"When David came to Mahanaim, Shobi son of Nahash from Rabbah of the Ammonites, and Makir son of Ammiel from Lo Debar, and Barzillai the Gileadite from Rogelim brought bedding and bowls and articles of pottery. They also brought wheat and barley, flour and roasted grain, beans and lentils, honey and curds, sheep, and cheese from cows' milk for David and his people to eat. For they said, 'The people have become hungry and tired and thirsty in the desert.'" - II Samuel 17:27-29

The adash is also mentioned in another list of food produce. In this example God gave instructions to Ezekiel to collect and store adash to be used later for bread: "Take wheat and barley, beans and adash [lentils], millet and spelt; put them in a storage jar and use them to make bread for yourself. You are to eat it during the 390 days you lie on your side." - Ezekiel 4:9

One of the more well known stories in the Old Testament concerns a stew made by Jacob that was given to his brother Esau in echange for this birthrights. This stew was made with adash (lentil) seeds.
 
"Once when Jacob was cooking some stew, Esau came in from the open country, famished. He said to Jacob, 'Quick, let me have some of that red stew! I'm famished!' (That is why he was also called Edom.)
     Jacob replied, 'First sell me your birthright.'
     'Look, I am about to die,' Esau said. 'What good is the birthright to me?'
     But Jacob said, 'Swear to me first.
     So he swore an oath to him, selling his birthright to Jacob.
     Then Jacob gave Esau some bread and some nazid adashim (lentil stew). He ate and drank, and then got up and left.
     So Esau despised his birthright."
- Genesis 25:29-34

This is an edited excerpt from Alvin Johnson's iBook 'Biblical Flora', 2017. The book is available for free download on iTunes. A teacher's edition is also available for purchase.