Campaniacum is the etymon inferred from numerous toponyms in France. The Toponymie générale de la France (TGF) derives it from a Roman personal name Campanius and the Gaulish suffix -acum. The -i- (which is important in the phonetic evolution of *Campaniacum) suggests that Campanius is a gens name.
The modern forms differ according to the diverse phonetic evolutions of the local dialects.
The initial /ka/ of *Campaniacum became /ʃa/ (written Cha) in most of Gaul, both in langue d'oïl dialects and the northern langue d'oc dialects; but north of the Joret line, and most Langue d'oc dialects (southern one) /ka/ (written Ca-) was preserved.
The form Champigny is purely French (f. e. : Champigny-sur-Marne is in the Val-de-Marne), because of its [i] between [p] and [ñ]. Gaston Zink offered an explanation for the sequence /ign/ in place of the expected /agn/: before the palatal consonant /ɲ/, the /a/ shifted to /e/, which in turn closed to /i/; Zink points out the parallel form (fungum) campaniolum ('mushroom') which became champegneul in Old French and champignon (with substitution of suffix) in modern French.