There is an enormous dispute about wether the word hussar derives from Hungarian, Serbian or Greek.

The "20" is one explaination (Hungarian)

Another is that it derives from the Serbian Gus-Gusar (I don't remember the exact meaning of the word).

A third is that it derives from the word "Hosarii", a Greek words which means "ambushers" and describes a light cavalry type of Byzantine troops (from 8th century AD) who served as border patrol and militia. Those were recruited from the Balkan provinces of the empire (mainly Greece-Serbia).

There is an even older Roman term, but I can't remember it right now...