As the Lord liveth; he swears by Jehovah; either because he did acknowledge their Jehovah to be a God, being, it may be, convinced and instructed therein by David, though he did worship Dagon with him, and above him: or because this was David's God, and therefore he swore by him; partly out of complaisance with David, that he might receive his unwelcome message to him with less offence; and partly that this oath might gain more credit to his words with David. Thy going out and thy coming in with me, i.e. thy whole conversation with me. See 1 Samuel 18:13, and many other places where that phrase is used. Since the day of thy coming unto me; though before that time there was evil in thee towards me and my people.
Related Commentaries of 1 Samuel 29:6
1 Samuel 29:6
6 Then Achish called David, and said unto him, Surely, as the LORD 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 lordsa favour thee not.