What does the idiom "snake in the grass" mean?

Question:

Literary Expression:

Literally, the phrase "snake in the grass" refers to an actual snake moving or sunning in the grass. The idiomatic expression, which takes on a different meaning, has been used as early as around 37 BCE by Roman poet Virgil. Italian poet Dante and American writer Mark Twain have also used the phrase.

Answer and Explanation: