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 )

Source: A King James Dictionary. (Used with permission. Copyright © Philip P. Kapusta)

Bibliography Information

"Entry for 'Forswear'". A King James Dictionary.