As noted in Francis Grose’s A Classical Dictionary of the Vulgar Tongue (Bookshop|Amazon), the term red rag, also redrag, is an old slang term for “the tongue,” as in the quotation he cites with a variant spelling of potato: Shut...
slob n.— «I heard that 5deuce mob shit (not sure of the spelling). Theyz the hardest Bay Area shit ta ever come out, bar none. Although I only heard a few songs (East Side Red Rag, Fuck a Slob, Gang Related), theyz the shit...