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The M. K. Pump is when you flex your biceps like a meathead after you show off or act like you're the best. It's meant to make people jealous and show you're a total poser.
After I dropped a 50 on the table, I did my M. K. Pump and said, 'Check me out, fool.'
He flexed like a baboon after he got the last laugh on me.
I M. K. Pumped so hard, my arm looked like a sausage.