The natural reaction of your sphincter to contact suddenly and violently when you are embarrassed by something or —especially— FOR someone else.
"Did you see that TV show with the juggler who was performing for the Queen? Wow, I sure did have a sphincter-blink when he dropped all those balls!"
by Christopher Nisperos June 29, 2006
An involuntary response to an action or stimulus.
I do a sphincter blink every time I hear a car backfire.
by Herb N. Dictionary October 25, 2002