it means 'you stop that'
"Ou sispann ki," Jesse hissed back. " I will keep telling you this til I'm blue in the face, if that's what it takes. But you are not a waste of space, Jay. You are one of my favorite people on this floating rock, and I will darn near break myself in half to try to show you that. And I'll keep at it, and I'd bet my last dollar your friends and that boy of yours would too."