ton [ME] Ton is a variant of tun, both spellings being used for the container and the weight in the past. The senses were differentiated in the late 17th century, with tun limited to a ‘cask’. A ton was originally a term for the capacity for a ship, originally the volume of space occupied by a cask or wine. The metric tonne—1 000 kilograms—first appears in English in the late 19th century, adopted from French. A little ton was, in French, a tonel, source of the word tunnel [LME]. This sense developed in English but has since been adopted back into French.