The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with snake in grass
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.