1) When you cum in your boxer shorts, leaving them sticky. Usually caused by either having a wet dream or by jacking off.
2) Shorts worn by a females that are so tight they hug the girls bum cleavage, so it can be clearly seen. Sometimes also associated with cameltoes. So named as they appear to stick to the bum.
1) I dreamt about my girlfriend naked last night, but when I woke up, I found I had sticky shorts
2) Did you see that ass - she was totally wearing sticky shorts