O que significa "an old chestnut"?

Can you remember any idea, joke or subject that's been repeated so many times it is no longer interesting? In Brazil, we have several of them. For example, there's a joke that's told every time someone puts pave, a very famous dessert, on the table. If you're brazilian, you know what I'm talking about. Our grandparents tend to say things so many times we start to wonder if they're really true. For example, I wonder if there might be some truth in the old chestnut that having ice cream while having a sore throat makes it worse.

If that's true or not, only science can tell me. But what really matters to us now is the expression in bold text I just used: old chestnut. Literally, a chestnut is what you see on the picture above. Figuratively speaking, an old chestnut is everything that's been repeated so many times it stops being interesting. For my entire life, I've been told having ice cream while sick makes the situation even worse. That myth became so boring and repetitive I started to question it. It turns that old chestnut is nothing but a myth since eating ice cream can even help soothe a sore throat. That's it for today guys. Take care!