An anastrophe of cum vōbīs (“with you, with ye”), from vōbīs pl (“you, ye”) (the ablative of vōs pl (“you, ye”), the plural of tū sg (“you”)), and cum (“with”) where cum is always followed by a personal pronoun. Compare with mēcum (“with me”), derived from cum and mē (“me”), tēcum (“with you”), from cum and tē (“you”), sēcum (“with oneself”), from cum and sē (“oneself”), nōbīscum (“with us”), from cum and nōbīs (“us”).
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 nōbīs (“with us”) would sound too much like cunnō bīs (“twice in the cunt”), so the words were reversed. This reversal was then applied to cum vōbīs, cum mē, cum tē, and cum sē.
A modern explanation is that the word ordering comes from the fact that Proto-Indo-European *ḱóm (from which cum derives) was an adverb, not a preposition as it became in Latin. As such the *ḱóm 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 nōbīscum (and mēcum, etc.) evolved into an adverb in its own right.
- (Classical) IPA(key): /u̯oːˈbiːs.kum/, [u̯oːˈbiːs̠kʊ̃ˑ]
- (Ecclesiastical) IPA(key): /voˈbis.kum/, [voˈbiskum]
vōbīscum (not comparable)
- Vulgar Latin: vōscum (see there for further descendants)