What does 대우탄금 (對牛彈琴) literally mean?

 ANSWER: facing a cow and plucking an oriental harp

대우탄금 (對牛彈琴) can literally translate as "facing (對) a cow (牛) [and] plucking (彈) an oriental harp (琴)." The expression is used to refer to something done for someone who does not have the intellectual ability or desire to understand or appreciate what is being done, so the effort is a waste of time. Here is an example sentence from a Korean book entitled "고사성어 대백과," which can translate as "Encyclopedia of Old Chinese Character Idioms."

아무리 좋은 강의도 그것을 이해할 수 없는 학생들에게는 대우탄금이랄 수 밖에 없다.

No matter how good the lecture, it is a waste of time if the students cannot understand it, like plucking a harp in front of cows.

The expression 대우탄금 is not in my Korean-English dictionary, which suggests that it is not a commonly used expression, but most Koreans have still probably heard of it. It is a 4-character Chinese idiom based on an old story. Koreans refer to such idioms as 고사성어 (故事成語), which literally means "ancient (故) incident (事) created (成) phrases (語)." If a 4-character Chinese idiom is not based on an old story, then Koreans simply refer to them as 사자성어 (四字成語), which literally means "four (四) character (字) created (成) phrases (語). The story behind the 대우탄금 idiom is as follows:

A man in the ancient Chinese state of Lu (노국 魯國) wanted to do something nice for his hard-working cow, so he started playing his harp in front of the cow, but the cow showed no sign of interest and just continued to eat the nearby grass. The man then made the sound of a mosquito (모기의 울음소리), which caused the cow to swish its tail. After that, the man made the sound of a crying calf (송아지의 울음소리), which caused the cow to raise its ears. This showed the man that the cow was smart enough to understand and appreciate the sounds of a mosquito and a crying calf but not the sounds of a harp. 

Here is a very good Korean-language video explaining the Chinese idiom 대우탄금 and the story behind it.