Green’s Dictionary of Slang

(US) untrue, false, usu. in journalistic context.

1849		N.Y. Daily Express 17 Feb. n.p.: As many of our citizens who intend to go to California may base their arrangements upon the mung news of some of the papers, we conceive it to be our duty to state that most of these letters are fictions [F&H].
1859		C. Mackay Life and Liberty in America 103: Among the pure Americanisms may be cited the following: [...] Mung, sham, false, pretended; mung news, a fabrication.