A misinterperetaion of Afro-American dialect (Shut up, lyin'). Used to express disbelief or sheer irritation.
"I just had sex with Lindsay Lohan."

"Shut up, lion."

or

"Did you translate those intelligence reports yet, Al?"

"Shut up, lion."
by not mike ditka October 29, 2004
Get the shut up, lion mug.