early 15c., "tax or duty on wine imported in tuns," from ton (n.1) + -age, and from Old French tonnage "duty levied on wine in casks" (c. 1300). Meaning "carrying capacity of a ship" is from 1718.
Entries linking to tonnage
ton n.1
"measure of weight," late 14c. The quantity necessary to fill a tun or cask of wine, thus identical to tun (q.v.). The spelling difference became firmly established 18c. Ton of bricks in the colloquial figurative sense of what you come down on someone like is from 1884.
-age
word-forming element in nouns of act, process, function, condition, from Old French and French -age, from Late Latin -aticum "belonging to, related to," originally neuter adjectival suffix, from PIE *-at- (source of Latin -atus, past participle suffix of verbs of the first conjugation) + *-(i)ko-, secondary suffix forming adjectives (see -ic).