1
general::
A treacherous person, as in Ben secretly applied for the same job as his best friend; no one knew he was such a snake in the grass. This metaphor for treachery, alluding to a poisonous snake concealed in tall grass, was used in 37 B.C. by the Roman poet Virgil (latet anguis in herba). It was first recorded in English in 1696 as the title of a book by Charles Leslie.
American Heritage Idioms
2
general::
Phrase(s): snake in the grass
a sneaky and despised person. • How could I ever have trusted that snake in the grass? • John is such a snake in the grass.
McGrawhill's American Idioms And Phrasal Verbs