The town is said to have been founded by one Chhaju and Chajunagar was curropted to Jhajjar. It is also derived from Jhaarnaghar, a natural fountain. A third derivation is from Jhajjar a water vessel because the surface drain-- age of the country for miles around runs into the town as in to a sink. The town existed in the times of Firoze Shah Tughlak, who had a canal dug from Sutlej to Jhajjar. It is now an important tehsil of Rohtak district.