The word 'tonneau' entered English directly from French in the early 20th century, where it originally meant a large cask or tun. This French term itself derives from the Old French 'tonel', a diminutive of 'tonne', meaning a large barrel. The automotive sense emerged from the visual resemblance of the early, rounded, open-topped rear passenger compartments of cars to a barrel lying on its side. Thus, the word's journey from a container for liquids to a part of a vehicle is a clear example of a metaphorical shift based on shared shape.

Structure: tonneau [Unsegmentable]. The word is a direct loanword from French.