ullage
To gauge the amount of empty space between the top of a cask and the level of liquid inside it.
The Duties on Spirits in England are charged on the number of proof gallons they contain, which is ascertained by first “gauging” or “ullaging” the liquor, and then “trying its strength” by Sike's hydrometer, in the way already described […]
Where it comes from
From Middle English ulage, from Anglo-Norman ulliage, from *ullier (“to fill a partially empty cask”), from Old French oel (“bunghole”, literally “eye”), from Latin oculus (“eye”). See French ouillage.
Adapted from the Wiktionary entry ullage (CC BY-SA 4.0).