'Frop (short for habafropzipulops) is the herb that "Bob" smokes in his pipe and the reason he is grinning. Technically its name is "habifropzipulops mariphasa lupina". It only grows in Tibet under very special circumstances. It grows by and blooms in moonlight. It only gets you high if it's the kind that's grown on Yeti droppings, or the graves of Tibetan holy men, or both. Many SubGenii enjoy smoking frop as a way to get their Slack back from the Conspiracy.
Here is a video of Rev. Sinphaltimus Exmortus using Time Control to smoke frop in his pipe with time going backwards (look at the direction the smoke goes in) - this shows some of the magical powers of frop: