This is one of the words I put in the too hard basket when I did a bulk acceptance of common-to-rare requests a while ago. The reason for deferring was that some of the comments in this thread questioned the idea that it isn't a common word.
The word is found a lot more often in British publications than American. One reason is that quite a few British regiments seem to have the word Hussar in their names. I also saw a few references to a Hungarian restaurant in Soho (London) called the Gay Hussar. From South African sources there were references to the Hussar Grill restaurant. Australian references tended to refer to some tropical fish, the pink hussar and the brown hussar - said to be delicious, but not well known in southern states.
When I did find a number of instances in a US corpus, most of them were from either Military History magazine or the Hudson Review, which ran some extracts from classic Russian novels. (The word appears 190 times in the Project Gutenberg version of War and Peace.)
My conclusion is that the word hussar is well known in various specialist contexts, but is not really common. Some people, like MK - and me too - will have encountered the word in reading without ever knowing its precise meaning. But I imagine quite a lot of players, especially those outside the UK, would not have come across the word.
So from now on it will be treated as a rare word.