The rule that if you listen very seriously and intently, with a deeply caring expression, to anyone until 4:30 a.m., they then must have sex with you. Sometimes a verb as well. Comes from the original, good version of Bedazzled, with Peter Cook (as the devil) and Dudley Moore (as the guy being tempted).
"Have you thought of just applying the rule of half past four?"
"What is it?"
"If you can stay wide awake and gaze at her thinking 'You are fascinating,' the whole time she talks, for everything she says, no matter how pointless and no matter how stupid, till half past four in the morning, you're in." (Not an exact quote)
I wanted her so bad that one night when she was drunk and dumping all this self pity, I kept telling her what an interesting person she was, until I finally half past foured my way into her pants.