The word in the original refers commonly in the Old Testament to a grain offering made to God (e.g., Genesis 4:3, Genesis 4:4, Genesis 4:5; Exodus 30:9; Leviticus 7:37; Isaiah 66:20). This tribute of grain is a sizeable one: it was so large it had to be carried by several men (see Judges 3:18). So Eglon is getting fat off Israel’s tribute.
15 Then the people of Israel cried out to the LORD, and the LORD raised up for them a deliverer, Ehud, the son of Gera, the Benjaminite, a left-handed man. The people of Israel sent tribute by him to Eglon the king of Moab.