Years ago they ate the hearts of a slain enemy
We enemies but your hatred could never enter me

from Nas – Stay Lyrics on Rap Genius

Meaning

The Aztecs and other mesoamerican civilizations would often remove the hearts of enemy warriors, collect the blood and handle their bodies as an act of sacrifice to their gods. Their hearts were often eaten as well, and are viewed as special.

Nas is also saying, even though they would eat their hearts, which in Nas' enemy’s instance is filled with hate, that that hate in his heart couldn’t enter him spiritually and emotionally (even if he was to eat the heart physically). It’s sort of a play on references and anatomy.

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