The Spanish word 'tonel' (meaning 'barrel' or 'cask') traces its origins back to the Celtic word 'tunna', which meant 'skin' or 'wineskin'. This Celtic word was borrowed into Late Latin as 'tunna', where its meaning evolved to specifically refer to a 'cask' or 'barrel'. The word then made its way through Old French, first as 'tonne' meaning 'large barrel', and then as 'tonel', before finally entering Spanish with its current form and meaning.

A related Spanish word is 'tonelada', which means 'ton' or 'metric ton'. This connection makes sense historically, as a 'ton' was originally a large barrel or cask used for wine storage, and later became a unit of measurement. The diminutive form 'tonelito' is also sometimes used for a small barrel.

English speakers might recognize the connection to several familiar words: 'ton' and 'tun' both share the same Celtic ancestor as 'tonel'. A 'tun' was historically a large wine cask, and the unit of measurement 'ton' originally referred to the weight or volume of what such a barrel could hold. Even the word 'tunnel' is related, as it originally referred to a barrel-shaped passageway, showing how the shape of these containers influenced our language.