The Collaborative International Dictionary
Bead proof \Bead" proof`\
Among distillers, a certain degree of strength in alcoholic liquor, as formerly ascertained by the floating or sinking of glass globules of different specific gravities thrown into it; now ascertained by more accurate meters.
A degree of strength in alcoholic liquor as shown by beads or small bubbles remaining on its surface, or at the side of the glass, when shaken.
Wiktionary
n. 1 (context distilleries English) A certain degree of strength in alcoholic liquor, as formerly ascertained by the floating or sinking of glass globules of different specific gravity thrown into it, and later by more accurate meters. 2 A degree of strength in alcoholic liquor as shown by beads or small bubbles remaining on its surface, or at the side of the glass, when shaken.