The ton is derived from the tun, the term applied to a cask of the largest capacity. This could contain a volume between 175 and 213 imperial gallons, which could weigh around 2,000 pounds and occupy some 60 cubic feet of space.

The tun (Old English: tunne, Latin: tunellus, Middle Latin: tunna) is an English unit of liquid volume (not weight), used for measuring wine, oil or honey. It is typically a large vat or vessel, most often holding 252 wine gallons, but occasionally other sizes were also used.

Edit: Copied from wikipedia.