secum

From Latin sē (the ablative of suī) + cum (“with”).

Two, not necessarily mutually exclusive explanations have been offered:

The first explanation was offered by Cicero (Orator §154), 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 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 *ḱom 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.

(Can this⁽⁺⁾ etymology be sourced?)

sēcum (not comparable)

1. with itself, with himself, with herself, with itself, with themselves

   Duxit secum virginem.

   He led the girl with himself.

   Pompeius a me petiit ut secum, et apud se quotidie essem.

   Pompeius requested me to be with him and at his house every day.

   Filium perduxere illuc secum, ut una esset, meum.

   They took my son along with them in their company thither.

   Amœnitates omnium Venerum atque venustatum is secum adfert.

   He brings all kinds of pleasures with him.

• Asturian: sigo
• Emilian: sêg
• Italian: seco
• Old Portuguese: sigo
  • Galician: consigo
  • Portuguese: consigo (com + sigo)
• Spanish: consigo

• mēcum
• tēcum
• nōbīscum
• vōbīscum
• quōcum
• quācum
• quibuscum
• quīcum

• secum in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
• secum in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
• secum in Gaffiot, Félix (1934) Dictionnaire Illustré Latin-Français, Hachette
• Carl Meissner; Henry William Auden (1894) Latin Phrase-Book‎, London: Macmillan and Co.
  • (ambiguous) circumstances demand: tempus (ita) fert (not secum)
    
  • (ambiguous) to think over, consider a thing: secum (cum animo) reputare aliquid
    
  • (ambiguous) to think over, consider a thing: considerare in, cum animo, secum aliquid
    
  • (ambiguous) to contradict oneself, be inconsistent: secum pugnare (without sibi); sibi repugnare (of things)
    
  • (ambiguous) to live to oneself: secum vivere