Hilary is the medieval English form of the Latin name Hilarius which is derived from hilaris meaning “cheerful.” It is also said to be from a Greek name Ιλαροσ (Hilaros) which also means “cheerful.” In ancient Roman tradition, the Hilaria were a series of festivals celebrated during the vernal equinox in honour of the goddess Cybele.
The name was borne by St. Hilaire of Poitiers (c.300-c.368) a Catholic theologian and bishop who is referred to as the Hammer of the Arians (Malleus Arianorum) as well as the Athanasius of the West. It is borne by several other male saints including, St. Hilaire of Arles (403-449), St. Ilaro of Galatea (476-558). It is also borne by Hilarius an English poet who wrote in Latin (1125). It is name the name of a few places, one is a town in Cornwall and the other is a village in Glamorgan Ireland, both of which were named for St. Hilaire of Poitiers.
The name was used as a male name all the way up to the early 20th-century (this does not stop the name from being unusable for a male 😉 It was especially popular in Medieval Britain. In the 1930s, the name seemed to have become prevalent among females, though its true feminine version should be Hilaria in English. It is borne by former First Lady and current Secretary of State Hillary Rodham Clinton (b.1947). Other forms of the name include (male versions first):