By ‘callow’ I here mean ‘inexperienced, immature’.
After some extensive delving in some scripts of yore, I found the seguent thoughts on the etymology of the name ‘Belarus’:
According to one unproven interpretation Byelorussians (or Belorussians) are given that name because of their white or grey rough upper part of clothes they used to wear.
Second (and newer) opinion says that is comes from an old Russian expression ‘bjelyj’ which means ‘free from giving’ (tax to Tatars; Tatars didn’t conquer Belarus because of the territory’s features).
Enjoy today’s etymology! 8)