sphincter blink 

An involuntary response to an action or stimulus.

I do a sphincter blink every time I hear a car backfire.

sphincter blink by Herb N. Dictionary October 25, 2002

sphincter-blink

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!"

sphincter-blink by wordbird June 29, 2006