The phrase itself is a bit of a cliché! It's been used for centuries, and its meaning is often understood even without a full explanation.
Here's an example:
"It's a hoary old chestnut, but it's true: time flies when you're having fun."
In this case, the phrase "time flies when you're having fun" is considered a hoary old chestnut because it's a very common saying that's been used countless times.
Other things that can be called "hoary old chestnuts" include:
* Jokes that have been told and retold so many times they're no longer funny
* Stories that have lost their impact due to constant repetition
* Arguments or opinions that are considered tired and outdated
The phrase "hoary old chestnut" is often used in a playful or humorous way, suggesting that the listener is aware of the cliché but doesn't mind sharing it anyway.