A correspondent in Notes and Queries (March 5th, 1892) suggests as the fons et origo of this word the Italian Uomo bugiardo, a lying man.
To hum used to signify “to applaud,” “to pretend admiration,” hence “to flatter,” “to cajole for an end,” “to deceive.”
“He threatened, but behold! `twas all a hum.”
Peter Pindar i. 436.
“`Gentlemen, this humming [expression of applause] is not at all becoming the gravity of this court.” —State Trials (1660).
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894