Where does “*কঙ্কড়া” come from?

*কঙ্কড়া (Old Assamese) comes from Sanskrit कर्कट, from Proto-Indo-Aryan karkartas.

Ancestry of “*কঙ্কড়া”, step by step

Step	Language	Word	Meaning
1	Sanskrit	कर्कट	crab; the sign Cancer; a particular bird
2	Proto-Indo-Aryan	karkartas	—