Where does “lunacy” come from?

Lunacy comes from Middle English lunatik, from Latin lunaticus meaning moon-struck, ultimately from Proto-Indo-European lewk- meaning to shine.

lunacy (English): The state of being mad, insanity; Something...

Definitions

Step	Language	Word	Meaning
1	English	lunatic	An insane person; Crazed, mad, insane, demented
2	Middle English	lunatik	lunatic
3	Old French	lunatique	—
4	Latin	lūnāticus	—
5	Latin	lūna	—
6	Old Latin	losna	—
7	Proto-Italic	louksnā	—
8	Proto-Indo-European	lówksneh₂	moon
9	Proto-Indo-European	lewḱ-	lynx
10	Proto-Indo-European	lewk-	bright; to shine; to see

lunacy — Etymology, Origin & Word History | Etymology Explorer