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As indicated in the article, '혼전 또 혼전' 5강 싸움.. '이젠 며느리도 모른다' (Who will get a wild card in baseball, nobody knows), "며느리도 모른다." is broadly used in Korean to mean
Nobody can know (Nobody knows).
The noun '며느리' means
A son's wife, daughter-in-law.
I wonder when this expression started and how it got traction in Korean. Any special meaning in '며느리'?
It seems to have a modern origin. It showed up in a TV commercial of 고추장 in the 90s. In the commercial, the granny says even her daughter-in-law doesn't know the secret ingredient to her spicy rice cake dish. Such family secrets usually are passed on from mother-in-law to daughter-in-law, but the expression was to emphasize that it's even more secretive. This person in real life (who passed away) had a very successful restaurant business, and the business sign has the expression (google image search).
This expression from the commercial seems to have caught on to the entire South Korean society, and now is commonly used to mean 'secret'.
To double check, I did a quick search on the proverb list, and couldn't find a relevant entry containing '며느리'.
Traditionally the mother-in-law hands down the family secrets and traditions to the daughter-in-law, such as food recipes, home remedies, etc.
However, if one of these things is too important or difficult, the mother-in-law might want to put off teaching it to the daughter-in-law until later, and do it herself
Hence it is supposed to mean "Nobody knows" in the sense that, even the daughter-in-law, who is supposed to know all the secrets of the house, doesn't know it
http://www.hnews.co.kr/news/articleView.html?idxno=28775
벌써 오래전의 고추장 광고가 아직도 머리에 남아있다.
신당동 떡볶이 집에서 찍었는데 거기 할머니가 (떡볶이 양념고추장의 비밀은) "아무도 몰라, 며느리도 몰라" 이렇게 말해서 유명해졌다.
며느리가 시집을 오면 시어머니가 자기 집의 김장하는 법, 음식 만드는 법 등을 가르친다. 며느리는 시어머니가 가진 기술을 이어 받으며 시어머니의 기술을 습득한 유일한 사람이 된다.
시어머니는 단순한 기술이 아닌 중요한 기술 같은 것은 쉽게 가르쳐 주지 않으며 오랜 시간이 지난 뒤 며느리가 많은 배움을 얻었을 때 가르쳐 준다. 시어머니가 자신만이 할 수 있는 중요한 기술을 가지고 있을 때 며느리도 모른다는 말을 쓰게 된다.
Apparently it was popularized by a 고추장 ad at 신당동 떡볶이 집, as mentioned by Memming in his answer
The word 며느리 appears in the late Josun dynasty. The rice devoted to ancestors in traditional Korean family ceremony called 제사 used to be called 메. 며느리 was first made by adding this 메 to word 나리, so meaning that 'successor of the rice'. As time passed by, 메 became 며, and 나리 became 느리, finally becoming the word 며느리. The meaning of expression '며느리도 모른다' also derived form this tradition. The 'successor of the rice' usually worked in the kitchen, so if she doesn't know, then the senior one may not have told her about it, meaning that it's a secret.
