snake in the grass, n. — Green’s Dictionary of Slang

snake in the grass n.

[rhy. sl., either of which might prove a ‘treacherous friend’]

1. a looking glass, a mirror.

1859, 1860		Hotten Dict. of Modern Sl. etc.
1873		Sl. Dict.
1890–1904		Farmer & Henley Sl. and Its Analogues.
1933		Baltimore Sun (MD) 14 May 10/4: Snake in the grass — a looking glass.
1942		D. Runyon in Star (Marion, OH) 31 July 6/8: Rhyming slang is not founded upon allegory unless we except a few similes such as [...] ‘snake in the grass’ for ‘looking glass.’.
1947		Cheltenham Chron. 22 Feb. 5/3: ‘Snake in the grass,’ standing for looking-glass.
1960		J. Franklyn Dict. of Rhy. Sl.

2. a drinking glass.

2002