If I may quote Ryszart Brzezinski's excellent work, Polish Armies 1569-1696 (1) (Osprey, London, 1987).
"The hussar originated in Serbia towards the end of the 14th century. There are references to hussars in Poland in treasury returns of 1500, though they were probably in Polish service before this date. These early formations were foreign mercenaries, first known as Racowie from the term Rascia, 'Serbia', from the original centre of the Serbian state, Ras. The term 'hussar' probably originates not - as has been widely published - from any connections with the Hungarian husz meaning 'twenty', but from gusar, a Slavonic word meaning 'bandit'.