“ALMOST EVERY DAY I GO FOR A RUN down the bemerded pavements of North London,” Boris Johnson declared, announcing his candidature for London Mayor. Bemerded? We knew that he must mean fouled by dogs, but the word brought the reader up short for a moment, just as Johnson intended.

The word “bemerded” does not appear in the OED. Run it through Google and you get just 264 hits, most of them related to Boris himself, and the inquiry “Did you mean: bearded?” “Bemerded” appears in a translation of Rabelais, in a play by the weird occultist Aleister Crowley and a recent article by Christopher Hitchens. It has appeared in The Times only once in 222 years, as far as I can ascertain digitally, in a theatre review by Irving Wardle in 1989. Will Self managed to use it in 2001 when discussing the possible links between childlessness and avant-garde anomie: “It is hard to maintain the ultimate futility and purposeless of existence when you’re confronting a packet of wet ones and bemerded little bum.” That familiar old Nietzsche and the Nappies theory.

But mostly “bemerded” is a word that Boris has made his own. He has used it to describe the streets of Brussels, the streets of England, the streets of Islington and the Oxford cell floor where he spent the night after an evening boozing with the Bullingdon Club. And he was going to get it into his first official statement as mayoral candidate by hook or by crook. Rightly, for bemerded is his signature word, being at once slightly risqué in an antique way, gently self-mocking, and also rather clever.

Everyone should have a signature word.

For some years, mine was “gallowglass”, a word derived from Gallóglaigh, the ancient mercenary warrior élite of western Scotland. Military chiefs in medieval times would hire a gallowglass to act as an aide and bodyguard, to do his dirty work: as a Scottish outsider, the gallowglass could operate above local feuds with impunity. All powerful men need a gallowglass: Alastair Campbell was Tony Blair’s gallowglass; Karl Rove was George W. Bush’s.

words is going on to use them in everyday conversation,” writes Christopher Foyle, in the introduction to Foyle’s Philavery: A Treasury of Unusual Words. A philavery is “an idiosyncratic collection of uncommon and pleasing words”, and few are better placed to be philaverists (I think I may have made that one up) than Foyle, the third generation of his family to operate Foyle’s bookshop in Charing Cross Road.

Foyle’s collection is splendidly bizarre, running from “abacinate”, meaning “to blind someone by putting red hot metal before their eyes” (the only recorded use of which is the lyrics of the 1986 song Angel of Death by the American thrash metal band Slayer) to “zoonist”, “someone who believes that nature as whole or natural objects are living beings”.

It was General Norman Schwarzkopf, of all people, who inspired Foyle’s philavery. In the course of the first Gulf War, the US commander referred to some doubtful intelligence as “bovine scatology” – as a sensitive killing machine he could not bring himself to deploy the word bullshit. “From then on,” writes Foyle, “I started to make a note of any word I came across that I was unfamiliar with.” Actually, Schwarzkopf’s circumlocution is, in my opinion, the very opposite of a collectible word, since it seeks to make a simple term deliberately obscure. The best unusual words are those that say something in a new, evocative or colourful way.

“Crepuscular” (relating to twilight) is a good example of a little-used but most useful word: no other evokes so pleasingly the magical quality of light at dusk, although the Scottish word “gloaming” comes close. There is also a wonderful Hindi expression hawa khana, which means “breathing the air”, the moment at the end of a long, hot day when the earth exhales.

Foyle is particularly good on the subclass of unlikely words that sound as if they ought to be filthy and are not: futtock (one of the carved timbers that forms a rib in a wooden ship’s frame), inspissate (to thicken, condense), formication (the sensation, usually hallucinatory, that insects or snakes are crawling over the skin) and aprosexia (an abnormal inability to pay attention) – a condition from which I have been a lifelong sufferer.

The reverse type – words that sound neutral, even scientific, but turn out to be quite rude – are also to be treasured, such as lupanarian (pertaining to a brothel) and callipygian, a most beautiful term meaning to have a finely developed and well-proportioned bottom.

There is a tendency in public discourse to avoid uncommon words, for fear that they will sound pretentious. George Orwell’s prescription on writing simply has evolved into a refusal to write anything beyond the ken of the spell-checker, while politicians stick firmly to the well-trodden paths of vocabulary.

But before we hail Boris as the first political philaverist of modern times, let us pay tribute to John Prescott, who did not merely use words nobody else understood, but invented an entire language of his own.

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