Source : Webster's Revised Unabridged Dictionary (1913)

Langue d'oil \Langue` d'o["i]l"\ [F., language of o["i]l yes.]
   The dialect formerly spoken north of the Loire (in which the
   word for ``yes'' was o["i]l, F. oui).

Source : WordNet®

Langue d'oil
     n : medieval provincial dialects of French spoken in central and
         northern France [syn: {Langue d'oil French}]