mid-15c., "large stopper for a cask," of uncertain origin, perhaps from Middle Dutch bonge "stopper;" or perhaps from French bonde "bung, bunghole" (15c.), which may be of Germanic origin (or the Germanic words may be borrowed from Romanic), or it may be from Gaulish *bunda (compare Old Irish bonn, Gaelic bonn, Welsh bon "base, sole of the foot"). It is possible that either or both of these sources is ultimately from Latin puncta in the sense of "hole" (from PIE root *peuk- "to prick"). Transferred to the cask-mouth itself (also bung-hole) from 1570s.
updated on August 30, 2017