to clench your bum when you hear of something disgusting
Person 1 "Oh my did you hear about .....?? she was shaving her legs and while shaving she cut her leg that bad that you could see the bone"
Person 2"Oh my god that just made my bum wink!"
When something gives you so much fear that it makes your bum wink, also known as 'the closing effect'.
Holy shit-flaps Alessia! We almost got hit by that lorry! I was so scared, it made my bum wink!