Where does “hilarious” come from?

Hilarious comes from Latin hilaris, derived from Ancient Greek ἱλαρός, which itself comes from Ancient Greek ἵλαος.

hilarious (English): Very funny; causing great merriment and laughter;...

Definitions

Step	Language	Word	Meaning
1	Latin	hilaris	cheerful, lively, light-hearted, merry
2	Ancient Greek	ἱλαρός	cheerful, merry, joyous, glad, happy
3	Ancient Greek	ἵλαος	propitious, gracious, merciful; kind, mild,...
4	Proto-Indo-European	sēlh₂-	to calm, quiet, be favourable

hilarious — Etymology, Origin & Word History | Etymology Explorer