Law of Expectation
The universal truth that if you groom yourself in preparation for an expected hook-up, the hook-up will not occur immediately after. It only occurs when you haven't prepared yourself properly. Usually applicable to females.
For example, a girl shaves her legs in preparation for an expected hook-up but then the guy tells her he can't hang out that night.
She tells her roommate "Damn, I should've known I wouldn't get any tonight; its the Law of Expectation."
She tells her roommate "Damn, I should've known I wouldn't get any tonight; its the Law of Expectation."