'Swear' in the Bible

Now therefore arise, go forth, and speak comfortably unto thy servants: for I swear by the LORD, if thou go not forth, there will not tarry one with thee this night: and that will be worse unto thee than all the evil that befell thee from thy youth until now.

and Abishai son of Zeruiah giveth help to him, and smiteth the Philistine, and putteth him to death; then swear the men of David to him, saying, 'Thou dost not go out again with us to battle, nor quench the lamp of Israel.'