Cowboy, you calf like the back of my leg, you ain't seein' what I said
Lupe continues with the body part analogies to express his superiority in the rap game. Lu is a cowboy (metaphorically, of course) while these other rappers are calves, or young cows, in comparison. Saying “calf” and not cow may be a nod to the fact that Kendrick is relatively young in this rap game.
Cowboys catch calves at rodeos during “tie-down roping” so lyrically Kendrick is a calf and Lupe is the Cowboy roping him in and controlling him.
The calf is also a part of the back of the human leg.
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