A cautionary exclamation usually uttered after breaking wind in order to warn those around you of the imminent smell. Derived from the television show "Watch Out, Beadle's About!" in which chirpy bearded funnyman, Jeremy Beadle, played convoluted pranks on an unsuspecting member of the public.
Crispin: Watch out, everyone, Beadle's about.
Helen: Did a rat die up your arse again,
Crispin, coz THAT STINKS.
Crispin: I did warn ya!
Note: Deviations on the same theme are also possible, eg. simply using the word "Beadle" to denote a flatulent eruption - "Sorry guys, I've just dropped a Beadle"