The thug life tattoo is an often seen joke (typically displayed on an otherwise unassuming or mild-mannered character) to hint at there being a somewhat darker side to them. This seems to draw from the famous hip-hop artist Tupac Shakur, whose trademark stomach tattoo read "Thug Life"
This can be compared to Farnsworth's tattoo, which while on his back rather than his stomach, is of a very similar style.
While the tattoo's meaning is claimed to be an anagram for a more social message, it would also appear to extoll the virtues of the modern gangster lifestyle. The joke arises from the juxtaposition of this message with the personality of the character on whom it is emblazoned.