Charles Ellicott Commentary 1 Samuel 29:6

"Then Achish called David, and said unto him, As Jehovah liveth, thou hast been upright, and thy going out and thy coming in with me in the host is good in my sight; for I have not found evil in thee since the day of thy coming unto me unto this day: nevertheless the lords favor thee not." — 1 Samuel 29:6 (ASV)

Surely, as the Lord liveth. —This seems a strange oath for an idolatrous prince like the King of Gath to use—“By the life of Jehovah.” It was probably the equivalent of Achish's real oath, unless, as Keil supposes, David's Philistine friend, in his oath, used the formula he thought would be most acceptable to David, whom he regarded as falsely injured by the suspicion of the Philistine leaders.