Disputed. Generally believed to be from Gaulish/Transalpine Gaulishcaballos.. This term is cognate to Welsh ceffyl, Manx cabbyl, Scottish Gaelic and Irish capall.
Also compared is Ancient Greekκαβάλλης(kabállēs, “nag”), in turn possibly a borrowing from a Balkan, Anatolian, or northeast European language. Compare Turkish kaval, adjunct of at(“horse”), Proto-Slavic*kobýla.