Origin and history of ammo

1620s, "military stores and provisions," from French soldiers' faulty separation of French la munition, as if *l'amunition; from Latin munitionem (nominative munitio) "a fortifying" (see munition).

The mistake in the word perhaps was by influence of French a(d)monition "warning." The error was corrected in French (Modern French munition), but retained in English, with the unetymological double -m- spelling to conform to words in Latin. At first it meant all military supplies in general; in modern use it means only material used in the discharge of firearms and ordnance.

colloquial or slang suffix, attested by 1880s in bucko, later (mostly 1910s and 20s) also in wino, ammo, combo, cheapo, kiddo, neato, boffo, the names of the Marx Brothers, commercial products (Sterno, Wham-O), etc. OED (1989) reports it "widespread in English-speaking countries but nowhere more so than in Australia."