John Gill Commentary 2 Samuel 15:21

"And Ittai answered the king, and said, As Jehovah liveth, and as my lord the king liveth, surely in what place my lord the king shall be, whether for death or for life, even there also will thy servant be." — 2 Samuel 15:21 (ASV)

And Ittai answered the king, and said With an oath, as follows:

[as] the Lord lives, and [as] my lord the king lives ; which he took to confirm what he after says, and to put an end to the debate between them:

surely, in what place my lord the king shall be, whether in death orlife, even there also will your servant be ; signifying that he would attend him wherever he went, hazard his life in his cause, and live and die with him.