He changed his song; he altered his account or evidence. It was bought for an old song, i.e. very cheap. His morning and his evening song do not agree; he tells a different story.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Son of Prattlement * SooterkinNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes