The Cossack in Russia ,who are they?

I heard that Cossacks also acted like a sort of safety valve in Russian society for a long time, the most hot-headed/rebellious people would run off to become Cossacks and not cause trouble in Russia proper ;)

Also did anyone mention that they were very free and rebellious earlier on, but later they were subjugated and only then did they become known for being the shock troops of the Russian empire?

I think the WORDS kazar, kazak, cosak has comon origin. In slavic languages they meant mounted robber.
May be the words hussar, courser(eng.) has the same origin.

Kasars called themselves: sabir. (Konstantinos Porphirogenitos: sabartoi asfaloi)

Thr turkish word, which means 'free man' should give an other solution.