A phrase used in response to somebody doing or saying something so unattractive, that the most mighty of erections would quickly shrivel to the size of an acorn. Very much like the head of a tortoise would retract within its shell.
"Dude, did you see Lucy eating that sausage and egg mcmuffin earlier?"
"Yeah man, she got mustard up her nose and proceeded to spoon it out and eat it, total retracting tortoise."
When your butt/balls get burned, your penis will retract all the way into your throat, which will give you Retracting Genitaliation Syndrome. Also known as RGS.