Another way to say shut up. Confuses the person you tell this too which usually results in them being quiet. More effective than saying plain ol' shut up. The definition of a sphincter is essentially an opening in the human body (example: mouth)
Nick: Hi I'm Nick. I'm really fucking annoying and laugh at my own jokes. I have no sense of humor and look like a preteen girl. I will be a virgin forev...
Dan: SHUT YOUR SPHINCTER!!