What does 여반장 (如反掌) literally mean?

 ANSWER: like (如) turning upside down (反) the palm of your hand (掌)

My Korean-English dictionary defines 여반장 (如反掌) as "(being) very easy" and "as easy as falling off a log," but the phrase literally means "like (如) turning upside down (反) the palm of your hand (掌)," which is usually a very easy thing to do.

The Chinese character 反 (반) can mean "to oppose" or "to be against," but it can also mean 거꾸로, which can translate as "inside out" or "upside down."

From Dong-A's Prime Korean-English Dictionary