Where does “lon” come from?

lon (Middle Irish) comes from Old Irish lón, from Proto-Celtic *lawano-, from Proto-Indo-European lewH- — louse.

Ancestry of “lon”, step by step

Step	Language	Word	Meaning
1	Old Irish	lón	fat; provision, food, victuals, sustenance; feast
2	Proto-Celtic	*lawano-	—
3	Proto-Indo-European	lewH-	louse