The tongue. Shut your potatoe trap, and give your red rag a holiday; i.e. shut your mouth, and let your tongue rest. Too much of the red rag (too much tongue).
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.
Red Letter Day * Red Sail-yard DockersNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes