Where does “Londyn” come from?

Londyn (Polish) comes from Latin Londinium, from Proto-Celtic *Londinyom, from Proto-Indo-European lendʰ- — land, heath; loins.

Ancestry of “Londyn”, step by step

Step	Language	Word	Meaning
1	Latin	Londinium	London
2	Proto-Celtic	*Londinyom	—
3	Proto-Indo-European	lendʰ-	land, heath; loins

Londyn — Etymology, Origin & Word History | Etymology Explorer