I knew it, I been through hell but still I'm coolin
Young Roddy has been through many hardships in his life. He has seen a plethora of bad things happen growing up on the streets of New Orleans. It could have been anything from murder, to robbery, to jail, to fights, or even much more. For this line he is comparing these struggles as a form of Hell.
Idealistically Hell is a place that is engulfed in flames, but even though Roddy has been through that place (metaphorically speaking) he is still coolin' or taking it easy.
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