Originally, друг was the predicative (short) form of другой (“another”). It is related to второй ("second”) < OCS въторъ (“other”, "second”) < PIE *wi-tero- (“more apart”) < PIE base *wi- (“separation”) + comparative suffix *-tero- (“-er”).


друг (drug, 'm', друзья, druz’já, Kigezo:p) (ru)

  1. rafiki
      • close friend
      • very old friend
      • friend of the family
      • childhood friend
  2. (in pairs) other, another
      • one another; each other
      • to one another; to each other
      • one after another; in single file
      • against each other
      • one from another
      • face to face; vis-à-vis
      • with each other
      • at one another; from one another