To turn stag; to impeach one’s confederates: from a herd of deer, who are said to turn their horns against any of their number who is hunted.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Squirrel Hunting * To StagNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes