to take the name of god in vain

from The Collaborative International Dictionary of English v.0.48

Vain \Vain\, n.
   Vanity; emptiness; -- now used only in the phrase

   {in vain}.
      [1913 Webster]

   {For vain}. See {In vain}. [Obs.] --Shak.

   {In vain}, to no purpose; without effect; ineffectually. " In
      vain doth valor bleed." --Milton. " In vain they do
      worship me." --Matt. xv. 9.

   {To take the name of God in vain}, to use the name of God
      with levity or profaneness.
      [1913 Webster]