Source : Webster's Revised Unabridged Dictionary (1913)

Voir dire \Voir dire\ [OF., to say the truth, fr. L. verus true
   + dicere to say.] (Law)
   An oath administered to a witness, usually before being sworn
   in chief, requiring him to speak the truth, or make true
   answers in reference to matters inquired of, to ascertain his
   competency to give evidence. --Greenleaf. Ld. Abinger.