Etymology

From nobis "us" the ablative of nos "we", + cum "with".

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 ****", 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.