A silent fart. Used in UK slang from at least the 1980's. The theory was that butlers who were serving tea had to train themselves to fart without being noticed, but this may not be the origin of the phrase.
ORIGIN: Drawings of Richard & Tim Butler of the Psychedelic Furs lost and never to resurface, featuring lame ass 70s style lettering "DRUGS" (richard) and "BEER." (tim) with their looming heads.