Gas that a well off person passes. The word fart is too offensive for them.
Sir Charles passed some brown air that Sir Winfred walked in on.
Brown air is the kind of rectal expulsion that often causes symptoms similar to those experienced by mustard gas victims. An acrid, thick, almost tangible odour.
"Goodness me, Ponsonby just snuck out as I walked in only to be assailed by a cloud of the foulest brown air!"
"Yes, he's a bastard is Ponsonby. Notorious for his obnoxious air biscuits"