Unclear. No cognates in other Slavic languages. Possibly related to Ossetianкъæбула(k’æbula, “young dog”). Trubachyov hypothesizes that the term originally referred to fur color, and explains it as *ко-бѣль, ultimately from Proto-Slavic*bělъ(“white”).
Vasmer, Max (1964–1973), “кобель”, in Etimologičeskij slovarʹ russkovo jazyka [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Trubačev O. N., Moscow: Progress