1823, "cheerful," from Latin hilaris "cheerful, lively, merry, joyful, of good cheer" (see hilarity) + -ous. Meaning "boisterously joyful" is from 1835. Related: Hilariously.
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hilarity n.
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 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.
-ous
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."