I’ll make you sing O be joyful on the other side of your mouth; a threat, implying the party threatened will be made to cry. To sing O be easy; to appear contented when one has cause to complain, and dare not.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
OafNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes