mid-15c., from L. hilaritas (gen. hilaritatis) "cheerfulness, gaiety," from hilaris "cheerful, gay," from Gk. hilaros, related to hilaos "graceful, kindly." In ancient Rome, Hilaria (neut. pl. 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.