An idea or subject, usually flawed, which perpetually resurfaces in conversation despite having been discussed to death long ago.
A: If evolution is real then how come we never see monkeys turn into humans, huh?
B: Oh god, not that old chestnut...
A person who takes ones virginity.
"That old chestnut popped my friend Kristen."
"I'm going to bang a virgin so i can become an old chestnut."