pull the other one, it's got bells on
Definition from Wiktionary, the free dictionary
As an extension of the phrase "someone pulling my leg"—teasing or goading by jokingly lying. The implication is that one leg has been pulled, and the joker will have more fun with the other one due to the bells.
- (idiomatic, humorous) Expresses disbelief.
- Is he going to study integrals? Pull the other one—he can't even do basic multiplication.