• The dialect formerly spoken north of the Loire (in which the word for `yes` was oil, F. oui).
(from the article `French literature`) ...south, where the Latin spoken was less subject to change. The tongue spoken to the north of an imaginary line running roughly from the Gironde ... The langue d`oïl had a tradition of dance and spinning songs before the troubadours exerted by the mid-12th century an influence encouraged by, ...
Langue` d'oïl' [ French, language of oïl yes.] The dialect formerly spoken north of the Loire (in which the word for 'yes' was oïl , French oui ). 
[n] - medieval provincial dialects of French spoken in central and northern France
Langue d`oil French noun medieval provincial dialects of French spoken in central and northern France