Definition of "one flesh"

"This is reasonable and natural," returned Pathfinder; ". . A woman would be likely to follow the man to whom she had plighted faith, and husband and wife are one flesh."

1840, James Fenimore Cooper, chapter 15, in The Pathfinder

Mary: Have not I been the fast friend of your lifeSince mine began, and it was thought we twoMight make one flesh, and cleave unto each otherAs man and wife?

1875, Lord Alfred Tennyson, Queen Mary: A Drama, act V

We know Christ's saying of the married that they are one flesh!

1911, John Galsworthy, “A Christian”, in Inn of Tranquility