Cause they ain't ready for the front, fax to 'em
They can’t handle Lupe face to face, so it’s better if he faxed the lyrics to them or if they heard it online.
Putting on/up a front can mean lying, being fake or acting defensive.
Lupe brings the facts to them so they are forced to face reality.
“Ain’t ready for the front” could mean not ready for the frontline as in war. The frontline is generally considered to be the most dangerous place to be on the battlefield. They are not ready to do battle with Lupe.
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