What does 숙불환생 (熟不還生 ) mean?

 ANSWER: "It's cooked [and] cannot be uncooked."

I have read HERE that the above expression can be used when a host offers food to a guest, though I don't know if it is commonly used these days. For example, if a host were to offer a guest some food and the guest were to politely decline with something like, "No, I'm okay" (아니오. 괜찮아요), which is a common courtesy in Korea, the host could then say, "It's cooked (숙 熟) [and] cannot (불 不) be uncooked (환생 還生)," so we might as well eat it. It literally means "cooked (熟) [and] cannot (不) be returned to (還) [its] raw (生) [state]."

And what does 월월산산 (月月山山) mean? Answer: "[After my] friend (月月) leaves (山山)."

The Chinese character for "friend" is 朋 (붕), which is made up of two "bodies" or "fleshes" 月 (월), and the Chinese character for "to go out" or "leave" is 출 (出) which looks like two "mountains" (山) on top of each other, so the expression 월월산산 (月月山山) means 붕출 (朋出), which means, in this context, "[after my] friend (朋) leaves (出)."

In the linked article above, a wife asks her husband if she should serve dinner while her husband's friend is visiting. The husband replies by saying 월월산산 (月月山山), which means, "No, wait until after my friend leaves." The reason the husband told his wife to wait on serving dinner was that he was embarrassed by their lack of side dishes.