Albert Barnes Commentary 2 Samuel 18:21

"Then said Joab to the Cushite, Go, tell the king what thou hast seen. And the Cushite bowed himself unto Joab, and ran." — 2 Samuel 18:21 (ASV)

Cushi - “The Cushite,” a foreign slave, perhaps belonging to Joab, whom he did not hesitate to expose to David’s anger. If, however, it is a name, it must be rendered “Haccushi.” In the title to Psalm 7, “Cush, the Benjamite,” cannot mean this Cushi, since the contents of the Psalm are not suitable to this occasion.