The phrase was invented by Tupac Shakur in the early nineties. He (and his father Mutulu Shakur from prison) started a movement called “Thug Life” that was an attempt to bring order to streets and limit gang violence. The goal was eventually to radicalise these gangs into political soldiers that could police the streets like his parents in the black panthers did in the 60s and 70s. The bloods and the crips - mortal enemies - both signed up to Thug Life and truced in 1992, under Thug Life. The Code of Thug Life was the rules these gangs followed up until Shakurs death in 1996.

Now people use it as a bullshit phrase that is void of all original meaning. 

It’s actually an acronym:

The Hate U Give Little Infants Fucks Everyone