old chestnut

informal

Definition

› a subject, idea, or joke that has been discussed or repeated so often that it is not funny any more: I wondered whether there might, after all, be some truth in the old chestnut that one's school days are the happiest of one's life.Tedious and uninspiring

(Definition of old chestnut from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)