[1] : 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
see also:
in vain for vain in vain to take the name of god in vain 

Vain >> V Words
Vain, definition of term: Vain
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In Vain >> I Words
In Vain, definition of term: In Vain
in+vain_pag1.html