Beenuts are a type of wizarding snack food that fly around in your mouth and jiggle your fillings. Apparently eating beenuts and smart drinks do not mix well as the old rhyme goes:
Smart drinks then beer, never fear
Beenuts, then smart drinks, nasty fart stink
Apparently the smell of the gas of someone who's had beenuts and smart drinks is so unaturally wrong that it causes peoples' eyes to water and makes them forcefully choke and cough. Beenuts might be a play on peanuts.
Ferd had beenuts and smart drinks unintentionally the night before the group's effort to rescue J.G. and consequently caused horrible gas while he was a bagel. This, however, saved most of the group's lives as everyone who might have eaten them ran streaming from the confrence room.