Hillary is the American spelling, now often used in other English speaking countries too, of the name Hilary. This name developed in the Middle Ages from a late Latin boy's name Hilarius, which itself came from the Latin adjective hilaris, meaning "cheerful." This is also the origin of some modern English words like hilarious and hilarity - it's because it sounds so similar to the latter, that Hilary is often thought to mean "laughter," which is almost true but not quite.
The feminine form of Hilarius was Hilaria; both names died out when the medieval form emerged. Then, as it was well into the 20th century, Hilary was always a boy's name (partly because of the early Christian male saint, Hilarius.) In the mid 20th century it began to be given more to girls; for a while it was associated with both, but now we see it as almost exclusively a girl's name.
Probably the most famous Hillary today is the possible future Democratic candidate for the US Presidency, Hillary Clinton.
The feminine form of Hilarius was Hilaria; both names died out when the medieval form emerged. Then, as it was well into the 20th century, Hilary was always a boy's name (partly because of the early Christian male saint, Hilarius.) In the mid 20th century it began to be given more to girls; for a while it was associated with both, but now we see it as almost exclusively a girl's name.
Probably the most famous Hillary today is the possible future Democratic candidate for the US Presidency, Hillary Clinton.