If you see something utterly disgusting where instead of getting aroused, you get turned off.
an example of a cock shrink could be witnessing your parents having sex.
Megan: "ohh harder John! err uhh i mean Joey"
Joey: "I'm not in the mood anymore, thats a cock shrink."
Brian: "I need to go jerk off" *Walks to bathroom to jerkoff and sees dad on toilet* "uhhh nevermind, my cock shrank"