Why is the ninth month called September?
late 14c., tonne, unit for measuring the carrying capacity of a ship, originally "space occupied by a tun or cask of wine," thus identical to tun (q.v.). The two words were not differentiated until 1680s. The measure of weight is first recorded late 15c.; the spelling ton is from 1530s, and became firmly established 18c.
unit of weight in the avoirdupois system equal to 2,000 pounds (907.18 kg) in the United States (the short ton) and 2,240 pounds (1,016.05 kg) in Britain (the long ton). The metric ton used in most other countries is 1,000 kg, equivalent to 2,204.6 pounds avoirdupois. The term derives from tun, denoting a large barrel used in the wine trade and named from the French tonnerre, or "thunder," in turn named for the rumbling it produced when rolled. Ton came to mean any large weight, until it was standardized at 20 hundredweight although the total weight could be 2,000, 2,160, 2,240, or 2,400 pounds (from 907.18 to 1088.62 kg) depending on whether the corresponding hundredweight contained 100, 108, 112, or 120 pounds