Quick answer
Hilarius is a boy name meaning "cheerful".
Meaning and history
Latin name derived from 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.
Notes and context
Usage: ancient roman