Roman name derived from Latin
hilaris meaning
"cheerful". Alternatively, it could be derived from the Greek name
Ἱλαρός (Hilaros) also meaning "cheerful" (the Greek word
ἱλαρός was the source of the Latin word
hilaris).
Saint Hilarius was a 4th-century theologian and bishop of Poitiers. This was also the name of a 5th-century pope.