hilarious (adj.)

mid-15c., from Latin hilaritatem (nominative hilaritas) "cheerfulness, gaiety, merriment," from hilaris "cheerful, merry," from Greek hilaros "cheerful, merry, joyous," related to hilaos "graceful, kindly," hilaskomai "to propitiate, appease, reconcile,"and probably from a suffixed form of a PIE root *selh- "reconcile" (source also of Latin solari "to comfort").

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.

word-forming element making adjectives from nouns, meaning "having, full of, having to do with, doing, inclined to," from Old French -ous, -eux, from Latin -osus (compare -ose (1)). In chemistry, "having a lower valence than forms expressed in -ic."