tun (n.)

[measure of weight or capacity], late 14c., "the quantity necessary to fill a tun or ask of wine;" the word is identical to tun (q.v.) "large barrel for wine, ale, or other liquid," often one of definite capacity.

The "weight-and-capacity" sense was (eventually) given its own spelling. The spelling with -o- became established 18c. (OED says "from c. 1688"); it is attested from 14c (tonne), and, though not phonetic, may have been retained partly because of the prevalence of Old French tonne, Medieval Latin tonna in legal forms and statutes in England.

As the sense shifted from the container to the definite capacity of it, ton came to be used broadly in Middle English for a definite capacity of many things (timber, stone, ships, iron, flour, earth) but the capacities varied between the trades and in different places. Ton as a national, legally defined measure of weight (in U.S., 2,000 pounds, a short-ton) is attested from 1485.

Ton of bricks or brick in colloquial images of heaviness and severity is by 1884.

The Omaha WORLD has dropped down on the officials like a ton of brick and the WORLD generally makes itself felt when it drops. ["Omaha Daily World," Jan. 16, 1888].

The phrase in the literal meaning figured commonly in accounts of lethal workplace accidents.

1901, rounded rear part of an automobile, from French tonneau, literally "cask, tun" (see tun).