Meaning:
Lit. “a cossack” is a common way to describe an (over)confident man willing to challenge himself and others in order to demonstrate bravery. This term can be positive as well as negative and the line is quite thin, depending on the context.
The origin lies in the name of a part of Ukrainian population which also formed a part of Polish-Lithuanian as well as Russian armed forces in the early modern era. These people were known to be brave and daring but also rowdy and unruly soldiers, not willing to bow to any authority.
In the 2000s, the term “kozak” has been used to denote not only people but also daring actions or in general, anything that impresses you (similar to “sztos“). However, the verb “kozaczyć” (“to play a cossack”, more or less) is quite negative and is used for people who definitely overdo their bragging and only talk the talk.
Examples:
SpongeBob chce udowodnić, jaki z niego kozak, lecz przypadkowo przywołuje ducha nieustraszonego pirata – Latającego Holendra.
Jakimowicz najpierw kozaczył w necie, a później przegrał w sądzie. TEN wyrok nie spodoba się gwiazdorkowi TVP.
Ale masz kozak zdjęcie na profilowym.