Boffo the abstract noun. Boffo is a kind of headology, described by Pratchett as "the power of expectations"; the strength one gains from behaving exactly as someone expects you to. Boffo is introduced by the witch Eumenides Treason in Wintersmith as a means by which she ensures people take her seriously.
It gets its name from the Boffo Novelty and Joke Shop, no. 4, Tenth Egg Street, Ankh-Morpork, from which Miss Treason purchases most of her interior decorating supplies, the better to ensure that when people come calling they don't see what is really there (a tired, blind 111-year-old woman), but what they expect (a venerable, terrifying 113-year-old witch).
The idea of Boffo is also understood by those forward-looking wizards who have to make a living in the community: the premises of C.V. Cheesewaller, DM(Unseen) B Thau, BF, in Quirm, are liberally festooned with hanging stuffed crocodiles, dribbly candles, skulls, and "the usual wizardly paraphernalia". Quoth the raven dismissively says "They get it all out of a catalogue. Believe me, it all comes in a big box". Any bets as to whose catalogue?
As a concept, Boffo is hardly unique to the Discworld: Roundworld bling such as Rolex watches, Armani suits and the crowns of kings is all Boffo, only more expensive.