Cask comes from Middle French casque or Spanish casco ('helmet, cask, skull'), of uncertain ultimate origin, perhaps from Vulgar Latin *quassicare ('to break'). The historical trail runs through casco in Spanish. The history must be left partly open, preserving the uncertainty rather than forcing a single unsupported ancestry. Used as a noun, cask now means "a barrel-shaped wooden container for liquids", so the present definition can be read against the older forms and meanings. The same lexical family is also visible in derived forms such as casket and related forms such as casket. That uncertainty is part of the entry: the safest account is the attested form and the limited comparisons preserved with it.