Is it ensure, insure, or assure?
"large cask or barrel," late 14c., presumably on some perceived resemblance. The original liquid measure was 63 old wine gallons (by a statute of 1423); later anywhere from 100 to 140 gallons. Borrowed into other Germanic languages, oddly, as ox-head (cf. Dutch okshoofd, German oxhoft, Swedish oxhufvud).