mid-15c., from Latin hilaritatem (nominative hilaritas) "cheerfulness, gaiety, merriment," from hilaris "cheerful, gay," from Greek hilaros "cheerful, gay, merry, joyous," related to hilaos "graceful, kindly." In ancient Rome, Hilaria (neuter plural of hilaris) were a class of holidays, times of pomp and rejoicing; there were public ones in honor of Cybele at the spring equinoxes as well as private ones on the day of a marriage or a son's birth.