Where does “ज्वर” come from?

ज्वर (Hindi) comes from Sanskrit ज्वर, from Sanskrit ज्वर् — to be hot with fever or passion.

ज्वर (Hindi): fever

Definitions

Step	Language	Word	Meaning
1	Sanskrit	ज्वर	fever, feverish heat; heated, inflamed
2	Sanskrit	ज्वर्	to be hot with fever or passion

ज्वर — Etymology, Origin & Word History | Etymology Explorer