Two, not necessarily mutually exclusive explanations have been offered:
The first explanation was offered by Cicero, who believed that the normal word order of cum nobis "with us" would sound too much like cunno bis "twice in the cunt", so the words were reversed. This reversal was then applied to cum vobis, cum me, cum te, and cum se.
A modern explanation is that the word ordering comes from the fact that in Proto-Indo-European the word *ḱóm (from which cum derives) was an adverb, not a preposition as it became in Latin. As such the *kom could appear before or after the object pronoun since it was the object of the verb, not the object of a preposition. As these special particles evolved into prepositions this word order became archaic even though it was still commonly used. Thus the contraction nobiscum (and mecum, etc.) evolved into an adverb in its own right.
mēcum (not comparable)
- with me
- Indignabar mecum.
- I resented with myself.
- Vade mecum.
- Go with me.
- Dii mecum militant.
- The gods are in favor with me.
- Dalmatian: conmaic
- Italian: meco
- Old Leonese:
- Old Portuguese: migo, comigo
- Old Spanish:
- Spanish: conmigo (con + migo)
- mecum in Charlton T. Lewis & Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
- mecum in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
- mecum in Félix Gaffiot (1934), Dictionnaire Illustré Latin-Français, Paris: Hachette.