ullage:  Ullage denotes the amount of unfilled space in a wine bottle or barrel. It goes back ultimately to Latin oculus ‘eye’ (a distant relative of English eye), in the metaphorical sense ‘bung-hole of a barrel’. As the word passed into Old French as oeil, this meaning followed it, and it formed the basis of a varb ouiller ‘fill up a barrel to the bung-hole’. From this was derived ouillage, which English acquired via Anglo-Norman ulliage as ullage. => eye, ocular
"amount by which a cask or bottle falls short of being full," late 15c., from Anglo-French ulliage (early 14c.), Anglo-Latin oliagium (late 13c.), Old French ouillage, from ouiller "to fill up (a barrel) to the bung," literally "to fill to the eye," from ueil "eye" (perhaps used colloquially for "bung"), from Latin oculus (see eye (n.)).