"Thug Life" is an American Expression that describes a way of life for people who face many adversities and dificulties throughout life, but who never give up fighting for their goals and dreams. i.e. hoodrat living. Eventually became a meme used to describe dominance of any kind done with nonchalance, usually associated with American street thugs stereotypes.
Usually appeared with deal with it glasses (aka "Thug Life glasses" due to its usage), and it is a subset of MLG memes.