The Old and New Testaments define the word vain basically the same way. It means empty. The Hebrew word for vain goes a little deeper into meaning nothingness, emptiness, vanity, useless, or anything that disappoints the hope that rests upon it.
So vain, or vanity, is an empty and useless position, condition and phrase. When Moses received the Ten Commandments the first four are the sacredness of the person and name of the Lord God. Exodus 20:7
“You shall not take the name of the Lord your God in vain, for the Lord will not hold him guiltless who takes his name in vain.” So to “take the name of the Lord your God in vain” is to use it for an empty (“not good”) purpose.
Our culture has taken the name of the Lord in vain by the constant and insidious phrase of, “Oh, my god.” Or, “Oh, god”, or “For Christ’s sake,” etc. Now it is just abbreviated to OMG, but abbreviated vanity is still vanity. This is like saying that God is empty, useless and nothing. If one uses these terms to describe the Lord… I suppose it is correct: God is useless to that person. Because it is socially popular to make God vain does not make God vain, it makes those vain who take the “name of the Lord your God in vain.” OMG is a horrible habit and a gross vanity.
“Again it is written, ‘You shall not put the Lord your God to the test.’” Jesus