This name was used in England during the Middle Ages for both sexes. In medieval documents the female version appears as Ylaria, Yllaria, or Hilaria. Some sources believe that in the Middle Ages it was a corruption of Eulalia, introduced from Spain, rather than a female version of Hilary. Apparently, versions of Eulalia are Aulaire, Aulazia, Ollala, and Eulària - the last especially is suggestive of Hilaria, and the first of Hilary.Hilary fell out of use between the Reformation, when saints' names lost favour, though it was still borne by some boys, and by girls in the version Ellery (does this suggest the Eulalia theory, perhaps, or even a folk link with the old Welsh name Eleri?).When it was revived in the nineteenth century Hilary was mainly for boys, although Florence Nightingale's female cousin was called Hilary - perhaps her parents were influenced by the Italian name Ilaria on their travels.The English Admiral Barlow during Nelson's time had the middle name Hilare - it was his mother's name, so that was how Hilary was preserved in that family before its revival.
Latin form of the Greek name Ilaria(f)-Ilarios(m). From the ancient Greek (not Latin) word Ilaros, meaning cheerful, joyful. Famous bearers of the name: Ilarios, priest of the goddess Demetra in Elefsis, and Ilarios, a politician of Antiocheia.