Like smackin' them in the back of him and my antonyms is on 'cain

from Lupe Fiasco – SLR 2 Lyrics on Rap Genius

Meaning

It’s a play on the story of Cain and Abel from Bible:
In the story Cain hit (smacked) his brother Abel on the back to kill him. And so, the antonym of Cain is Abel, meaning Lupe is ABLE (which is a homophone and an anagram to Abel) to murder them.
RapGenius™ King of Lyrical Trickiness strikes again.

But this line goes further. It’s connected with the next one, where he mentions heroin. Cocaine also has form of powder, but heroin is much more addictive and has stronger effects. He’s also playing on the word “smackin” too. “Smack” being slang for heroin. It goes in with the drug motif.

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