The Third Commandment spoken by God to Moses on Mount Sinai as recorded in Exodus 20. (If you're Catholic or of certain Protestant traditions, it is the Second.) The name of God is holy, and "holiness" means something that is set apart and consecrated for special use. One ought to be conscious of what one is invoking whenever it is used; and to be conscious of God is to be fearful. Using God's name in vain is like lobbing a hand grenade. We tend to think of this commandment in terms of swearing, which hardly means what it used to anymore, swearing an oath on something. The idea way back then was that "swearing" upon the Lord in vain was to call upon God's almighty person to back up some misguided or selfish wish or half-baked plan of your own. This is a Bad Idea.
In Judaism, this idea developed into a belief that even speaking God's holy name (YHVH) was taboo. In the modern era one finds some reverent Jews refusing to even write the word "God", instead spelling it G-d.