Mudak is a slur used to describe Russians in a derogatory way. Similar to dumb ass, but very explicit.
The word Mudak is a racial slur that has been used historically to degrade and dehumanize russians. The word is still used by some people for this purpose, and it is seen as offensive and hurtful to russians. If you are unsure about whether or not it is appropriate to use the word Mudak, it is best to err on the side of caution and avoid using it in any situation.