1530s, "fortification, action of fortifying or defending" (a sense now obsolete), also "materials used in war," from French municion "fortification, defense, defensive wall" (14c.), from Latin munitionem (nominative munitio) "a defending, fortification, protecting," noun of action from past-participle stem of munire "to fortify," from moenia "defensive walls," related to murus "wall" (see mural). Female workers in British shell factories in World War I were called munitionettes.
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 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.