When you feel as though you have to take a shit but have not digested enough food to do so, resulting in a combination of stomach pains/cramps and feelings of nausea in the anus.
I had a sour butthole this morning but I decided to try and shit anyway, ending up on the toilet for almost an hour and accomplishing nothing.
The opposite of "Sweet Dreams". Telling someone to have Sour Nightmares means you don't want them to sleep well or have good dreams. Instead, you want their sleep to be bad, unsettling, or even terrifying.
Guy 1: "I'm going to bed."
Guy 2: "Sour Nightmares."
Guy 1: "Don't you mean Sweet Dreams?"
Guy 2: "No."