ammunition (n.)

1620s, "military stores and provisions," from French soldiers' faulty separation of Middle 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 spelling conformed to words in Latin. At first meaning all military supplies in general, in modern use only material used in the discharge of firearms and ordnance.