I used to tutor ESL to Asian (mostly Chinese and Vietnamese) students. The idioms of English were the hardest thing for them to understand.
"I laughed all the way to the bank"
"Over the top" (was asked by a student, "how can it be over the top when it is the top?")
"Well, I'll be a monkey's uncle"
"skeletons in your closet"
These things were very difficult to teach.
A little trivia.... The term "I'll be a monkey's uncle" was first said/published in a newspaper during the Scopes 'Monkey Trial' in 1925.