I won't tell if you don't
A stupid promise you make when you're trying to avoid getting in trouble, even though you're going to tell the whole world. It's like saying you're cool, but you're actually a total mess.
'I won't tell if you don't,' she said, and then told the entire class.
'I won't tell if you don't,' he said, and then told his crush.
She said, 'I won't tell if you don't,' and then posted it online.