Definition of pull the other one, it's got bells on in English English dictionary
A jocular expression used to express disbelief. An extension of the phrase, and response to having, 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
Arthur: I am,... and this is my trusty servant Patsy. We have ridden the length and breadth of the land in search of knights who will join me in my court at Camelot.