Where does “लंका” come from?

लंका (Hindi) comes from Sanskrit लङ्का — island; the citadel of Ravana in the Ramayana.

Ancestry of “लंका”, step by step

Step	Language	Word	Meaning
1	Sanskrit	लङ्का	island; the citadel of Ravana in the Ramayana

लंका — Etymology, Origin & Word History | Etymology Explorer