While not technically colloquialisms, my grandfather had some great sayings.
When someone was going to be surprised or upset by what they were going to hear or find out he'd say, "They're going to **** down both legs". If they were going to be really shocked or upset, "They're going to **** straight up their back".
One time we were visiting and it was mentioned that my then 5 year old was constipated and nothing was helping he suggested that we give her a tablespoon of epsom salt dissolved in a glass of water stating that, "within a couple hours she would be able to **** through the eye of a needle at 50 paces". We opted not to take that advice.
I'll have to think of a few more. Needless to say, conversations with this man were never boring.