She says, third leg from a legend is sheer heaven

from Nas – No Introduction Lyrics on Rap Genius

Meaning

This girl was just fucked by Nas and compares the feeling with heaven and paradise. The third leg is obviously Nas’s long black dick.

Sheer heaven, right ladies?

It’s also not the first time Nas has referred to his “third leg”; in “Be a Nigger Too”:

Wake up in the morning, shake my third leg in the toilet

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