Where does “जलडाकू” come from?

जलडाकू (Hindi) comes from Hindi जल, from Sanskrit जल, from Sanskrit जल् — to be sharp; to be stiff or dull.

जलडाकू (Hindi): pirate

Definitions

Step	Language	Word	Meaning
1	Hindi	जल	water
2	Sanskrit	जल	stupid, unintelligent, dumb; a stupid man, idiot;...
3	Sanskrit	जल्	to be sharp; to be stiff or dull

जलडाकू — Etymology, Origin & Word History | Etymology Explorer