The Third Commandment is, “You shall not take the Name of the Lord your God in vain: for the Lord will not hold anyone guiltless, that takes his Name in vain”.  “To take” the Name of God means to speak it, and “in vain” means unnecessarily, idly, frivolously.

Perjury

The chief way of breaking this Commandment, though not the only way, is by what is called perjury; that is, the breaking of an oath made, for example in a Court of Law.  When anyone is brought into court and accused of having broken the law, evidence is given by witnesses, and it is largely on this evidence that he or she is found innocent or guilty.   Each witness swears, that is makes a solemn promise, to speak the truth.