Etymology Dictionary

ULLAGE,  the unfilled part of a cask.  (F.,—L.,—Gk.)   'Ullage of a Cask, is what a cask wants of being full;' Phillips, ed. 1706.—O. F. eullage, a filling up, the act of filling up that which is not quite full (Roquefort).—O. F. eullier, to fill a cask up to the bung; id.   The same word as Lyonnais ouillier, olier, to oil, also to fill to the brim.   When a flask is nearly full, the people of the S. of France add a little oil to prevent evaporation; so that 'to oil' is also 'to fill up'; see Wedgwood.—O. F. oile, oil.—Lat. oleum.—Gk. ἔλαιον.   See Oil. [†]

ADDENDA

'Onofrier, in his Glossaire Lyonnais, commenting on the verb olier, ouiller, to fill to the brim, observes that in the South of France, when a flask is nearly full, they add a little oil instead of a cork to prevent evaporation, so that to oil a flask is equivalent to filling it to the brim.   In Provence oliar signifies to anoint with oil, and also to fill up a cask.'—Wedgwood.   And, in fact, we find in Cotgrave the following:  'oiellage de vins, the filling up of leaky wine-vessels; oeiller les vins, to fill up wine-vessels which have leaked.'