old chestnut idiom informal
老掉牙的话题（或想法、笑话）
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. 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。