I won't tell if you don't
A cocky lie you tell someone to keep them from getting mad, even though you're totally going to tell everyone. It's like a promise made by someone who thinks they're the king of the hill.
'I won't tell if you don't,' he said, and then told the principal.
'I won't tell if you don't,' she said, and then told her mom.
He said, 'I won't tell if you don't,' and then he told his best friend.