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.
nypper * oafNathan Bailey's 1721 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes