Snake In The Grass Meaning

(idiomatic) A treacherous person.

Example: 1906, Horatio Alger, Randy of the River, ch. 6:
  "I trusted him too much from the start. He has proved to be a snake in the grass."
1914, William MacLeod Raine, A Daughter of the Dons, ch. 5:
  "Is he not here to throw us out—a thief, a spy, a snake in the grass?"
2008 Nov. 21, Bruce Crumley, "Which Woman Will Lead France's Socialists?," Time:
  Following her presidential defeat, Royal stunned many observers by publicly dumping Socialist Party leader François Hollande — her companion and the father of her four children — and announcing she'd seek his post during the current election. To some, that made Royal the symbol of the strong, modern woman in politics; to others, it cast her as the classic snake in the grass.