bill, we really don't have to resort to rocket science to use this word (and bridget* has the right spirit). it's a very old word whose original sense is:

1. The amount of wine or other liquor by which a cask or bottle falls short of being quite full (originally the quantity required to make good the loss by leakage or absorption).

1706 Phillips (ed. Kersey), Ullage of a Cask, is what such a Vessel wants of being full. 1833 Loudon Encycl. Archit. §1324 It is usually tunned into hogsheads of a hundred gallons each, leaving a few gallons ullage. 1835 Marryat Olla Podr. III. 297 (Moonshine), I held the bottle up to the candle to ascertain the ullage. 1885 W. Ecockes in Civilian 3 Jan. 141/2 A work+comprising tables of ullages of casks, whose bung diameters range from 15 to 40 inches.

and, at the risk of angryfying the anglo-saxon police, here's some *really early usage:

1297 Chanc. Misc. (P.R.O.) Bd. 2 No. 15 (5), Tradidi etiam eidem vnam pipam pro oliagio predictorum doleorum.


when I used this as the daily wwftd, a subscriber wrote to say that this is actually used in the restaurant where he works to refer to the dregs left over in glasses and bottles!