Shit's finna get thick again
Reference to the widely used term “the plot thickens”. In other words, shit’s about to get real now that Shady is back.
(This is also a nice subtle transitional metaphor for his dick that is continued in the next few lines.)
Knowing Eminem, this also is most likely about poop. When you get sick you usually have diarrhea, which is watery and obviously not thick. But now that he’s back he’s only sick with his rhymes, and his “shit” is back to normal.
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