1710, "allowance of space between the projectile and the diameter of the tube of a firearm," from wind (n.1) + -age. Meaning "allowance for wind deflection" is from 1867.
Entries linking to windage
wind n.1
"air in motion," Old English wind "wind," from Proto-Germanic *winda- (source also of Old Saxon, Old Frisian, Middle Dutch, Dutch wind, Old Norse vindr, Old High German wind, German Wind, Gothic winds), from PIE *wē-nt-o‑ "blowing," suffixed (participial) form of root *we- "to blow."
Normal pronunciation evolution made this word rhyme with kind and rind (Donne rhymes it with mind and Thomas Moore with behind), but it shifted to a short vowel 18c., probably from influence of windy, where the short vowel is natural. A sad loss for poets, who now must rhyme it only with sinned and a handful of weak words. Symbolic of emptiness and vanity since late 13c.
I have forgot much, Cynara! gone with the wind. [Ernest Dowson, 1896]
Meaning "breath" is attested from late Old English; especially "breath in speaking" (early 14c.), so long-winded, also "easy or regular breathing" (early 14c.), hence second wind in the figurative sense (by 1830), an image from the sport of hunting.
Winds "wind instruments of an orchestra" is from 1876. Figurative phrase which way the wind blows for "the current state of affairs" is suggested from c. 1400. To get wind of "receive information about" is by 1809, perhaps inspired by French avoir le vent de. To take the wind out of (one's) sails in the figurative sense (by 1883) is an image from sailing, where a ship without wind can make no progress. Wind-chill index is recorded from 1939. Wind energy from 1976. Wind vane from 1725.
-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).