I won't tell if you don't
A stupid lie you say to keep someone from getting angry, even though you're totally going to tell the whole world. It's like a promise made by someone who's got zero self-respect.
'I won't tell if you don't,' she said, and then told her best friend.
'I won't tell if you don't,' he said, and then told the whole school.
She said, 'I won't tell if you don't,' and then told her crush.