I'm aiming straight for your pelvis, you can't stomach me
The pelvis is the bone between the abdomen and the thighs.
If you can’t digest something, you’ll evacuate it through your back door (in the pelvic area), the “same way” you ate it (well not exactly but you get it). So basically, Kendrick is too good for you, he’s going straight through you; a clever way to say his lines goes way over your head. Also, Elvis was known for his pelvic “movement,” which ties in the previous line.
He used a similar line on the 2011 BET Cypher:
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