ammunition n.

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.