1 Samuel 28:2

And David said to Achish, Surely, since circumstances were of such a nature, thou shalt know what thy servant can do. David's answer was purposely evasive and ambiguous, but Achish took it to promise the action which he required, whereas David's answer really said, Conditions will determine what thy servant will do. And Achish said to David, Therefore will I make thee keeper of mine head forever, captain of his body-guard.