The odd word ordering on which this contraction is based comes from the fact that in Proto-Indo-European the word *kom (from which cum derives) was a special particle word, somewhat like 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.
- with us