This is a four-character idiom that literally means 'hard to encounter in a thousand years.' It describes an event or opportunity that is extremely rare and precious. You often see it before a noun like 机会 (opportunity) in a 的 structure, emphasizing that the situation is once-in-a-lifetime. Unlike the English 'once in a blue moon,' this idiom carries a stronger sense of historical or cosmic rarity — it's not for everyday things like rain or traffic.