1 Samuel 28:2

And David said to Achish, Surely thou shalt know what thy servant can do. And Achish said to David, Therefore will I make thee keeper of mine head for ever.

Surely thou shalt know what thy servant can do. This answer, while it seemed to express an apparent cheerfulness in agreeing to the proposal, contained a studied ambiguity, a wary and politic generality.

Therefore will I make thee keeper of mine head - or my life; i:e., captain of my body-guard-an office of great trust and high honour.