What does the Korean proverb "공자 앞에서 문자 쓴다" really mean?

ANSWER: "Pretending to know when one knows nothing, like loudly showing off what little one knows in front of a well-educated person like Confucius"

공자 (孔子) is the Korean pronunciation of "Confucius" (551 B.C. to 479 B.C.), the name of the Chinese philosopher. 앞에서 means "in front of," and 쓰다 means "to write."

The word 문자 (文字) has two meanings, one is "letters" or "characters" (글자) and the other is "idiomatic phrases from the Chinese classics" (한자 숙어 / 고사성어), so in the context of the above proverb, 문자 could probably mean either one, but I am going to use the second one.

So, "공자 앞에서 문자 쓴다" literally translates as "[He/She] is writing Classical Chinese idioms in front of Confucius."

But the meaning of the proverb does not really have anything to do with Confucius or any other "master." It is normally just used to sarcastically refer to someone who is using big words or difficult expressions to show off or to try to make people believe one is smarter than one really is. If you drop the "공자 앞에서" part and just say or write "문자(를) 쓴다," Koreans should know what you mean.

Here is the Korean definition of the proverb "공자 앞에서 문자 쓴다," as written on page 383 in the book shown below. The translation is mine:

"공자처럼 학식이 많은 사람 앞에서 조금 아는 것을 자랑삼아 떠드는 것 처럼, 아무것도 모르면서 아는 척한다는 뜻" 

"Pretending to know when one knows nothing, like loudly showing off what little one knows in front of a well-educated person like Confucius"