Belarus means 'White Russia', a name derived from the fact that this is the one part of Rus that, although conquered by the Mongols in 1240, was never settled by them. The term 'white' refers therefore to the purity of the people, who, unlike their Muscovite cousins, never intermarried with the Mongols.
https://www.lonelyplanet.com/belarus/narratives/background/history