Charles Ellicott 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. —Rather, the Cushite, probably an Ethiopian slave in Joab’s service, for whose incurring the king’s displeasure he had little concern.