a subject, idea, or joke that has been discussed or repeated so often that it is not funny any more老掉牙的话题(或想法、笑话)
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.老话说学生时代是人一生中最幸福的时光,我在想这话是否有些道理。