an old chestnut
A topic, saying, or joke that has been repeated so much that it has become boring or irksome. Whether there's truth in it or not, I can't stand that old chestnut "follow your heart."
an old chestnutor
a hoary old chestnutmainly BRITISH
COMMON If you describe something that is said or written as an old chestnut or a hoary old chestnut, you mean that it has been repeated so often that it is no longer interesting. Finally, how do you answer that old interview chestnut: `Why should I hire you?' The film is based on the hoary old chestnut of good twin/bad twin, separated at birth, final fatal meeting — you get the idea.
an old chestnuta joke, story, or subject that has become tedious and boring as a result of its age and constant repetition.
The most likely source for this sense of chestnut is in the following exchange between two characters, Zavior and Pablo, in William Dimond 's play Broken Sword ( 1816 ): ZAVIOR…When suddenly from the thick boughs of a cork tree— PABLO. (Jumping up) A chesnut, Captain, a chesnut…Captain, this is the twenty-seventh time I have heard you relate this story, and you invariably said, a chesnut, until now.