Its not the "slavic word". It is a bulgarian word that was later used by the Russian and Serbian states. It has no meaning in other slavic countries.

Redrawing of the epitaph of ichirgu boila Mostich. Translation (the title Tsar is enclosed): "Here lies Mostich who was ichirgu-boil during the reigns of Tsar Simeon and Tsar Peter. At the age of eighty he forsook the rank of ichirgu boila and all of his possessions and became a monk. And so ended his life." Now in the Museum of Preslav. In 705 Emperor Justinian II named Tervel of Bulgaria "Caesar", the first foreigner to receive this title, but his descendants continued to use Bulgar title "Kanasubigi". The sainted Boris I is sometimes retrospectively referred to as tsar, because at his time Bulgaria was converted to Christianity. However, the title "tsar" (and its Byzantine Greek equivalent "basileus") was actually adopted and used for the first time by his son Simeon I, following a makeshift imperial coronation performed by the Patriarch of Constantinople in 913.