[Wiktionary :] From Middle English with, from Old English wiþ (“against, opposite, toward”), a shortened form of wiþer, from Proto-Germanic *wiþr- (“against”), from Proto-Indo-European *wi-tero- (“more apart”); from Proto-Indo-European *wi (“separation”). [...]
[Etymonline, which appears to have extracted from OED :] [...] Sense shifted in Middle English to denote association, combination, and union, partly by influence of Old Norse vidh, and also perhaps by Latin cum "with" (as in pugnare cum "fight with"). [...]
History confirms that enemies (who are separate from, and who fight, each other) can become friends (who must be associated and united);
1. but what semantic notions explain, conciliate and underlie the polar opposites of association and opposition? Does my facile conjecture really explain the enormity of this semantic inversion?
- How did the Old Norse vidh and Latin cum influence the Semantic Shift of 'with'?