To swear falsely.
Again, ye have heard that it hath been said by them of old time, Thou shalt not FORSWEAR thyself, but shalt perform unto the Lord thine oaths: But I say unto you, Swear not at all; neither by heaven; for it is God's throne. ( Matthew 5:33 )
Bibliography Information"Entry for 'Forswear'". A King James Dictionary.