Elvas is a toponym of obscure origin, of which it can only be noted that in the Arabic writings of Al-Idrisi it appears written as elbax (12th century). See DOELP (s. v.), where the hypothesis that Elvas has its origin in the Roman anthroponym Elvius or Helvius is ruled out.

Cited bibliography: 

– DOELP = Machado, José Pedro. Dicionário onomástico etimológico da língua portuguesa. Lisboa: Livros Horizonte, 2003, 3ª ed.