Best candidate I can summon quickly to mind is obnubilate.
rarify (rarefy)
I believe the winner must be crissate, which means (of a woman) ‘to move her buttocks in sexual intercourse’ or ‘to ripple the haunches’. The only example of its usage that I’ve ever seen was in Sir Richard Francis Burton’s translations of medieval Arabic erotica. A cognate can be found in some English dictionaries, the noun *crissum *‘the feathers under the tail of a bird’, derived from Latin crissare (of a woman) ‘to move the haunches in copulation’.
So what you are saying is that twerking can lead to crissation.
a common word that is quite rare would be ‘defenestrate’. you would think with all the windows and highrise that it would be used more often, but no.
as for checking the rarity, why not just use google? ‘crissate’ has 2000 results, compared to ‘twiddle’ with 391 000 and ‘catillate’ at 1410.
I think Defenestrate is probably the most commonly known but rarely used word.
I like this word-make sure to use "Crissate"at least once in 2014!
Also “hight.” Don’t forget “hight.”
“Twiddling” is what pedantic jackasses like me refer to as a fossil word. It virtually never appears outside the context of the aforementioned phrase.
Since there’s probably no objective answer to this, and people are mostly giving examples of obscure words, let’s move this over to IMHO.
Colibri
General Questions Moderators
And even more pedantic jackasses such as me might point out that fossilization does not equate to rarity.
Hie with me to Lyme Regis beach, and we shall toddle together amongst the teeming ammonites.
“Hie,” “toddle,” and “teeming” don’t count as fossil words. They’re both used as independent verbs and are not confined to specific phrases, and they’re used in literal (read: concrete) senses, not merely figurative (read: idiomatic) ones.
Shall we have a nerdy slapfight? I’ll be cheating, of course.
Sir, I have met my nerdy match, and I gracefully withdraw.
:: regretfully removes cesti ::
That’s what I came in to say.
Several years ago, an improv comedy group I used to watch had to act out three words provided by the audience in a courtroom-framed charades game. One of the words they chose was “defenestrate.” When that word came up, one of the “actors” yelled “I call Pete!”
So the two “guessers,” Pete and another guy, came out. The first two words passed uneventfully; then came “defenestrate.” Pete’s actor did a fine pantomime of opening a window and throwing himself out, and Pete came up with the word almost immediately.
To this day, that was the longest standing ovation I have ever been part of.
“Twiddle” is actually in current use in a narrow niche: In discussion of the card game Magic: the Gathering, to twiddle a card is to repeatedly cause it to become tapped or untapped (turned sideways, generally representing some sort of use of the card, or exhaustion of potential for use). This was originally used of the card Basalt Monolith, which could (among other uses) be tapped for the purpose of untapping itself, an effort which generally has zero effect and is thus equivalent to twiddling one’s thumbs, but it’s also been applied to a wide variety of other cards, often in contexts where it’s quite useful indeed.
And incidentally, I found at least two instances of “yclept” being used (as opposed to just mentioned, as it was in this thread) by people other than Skald on this message board, and there are probably more: Akatsukami and choie.
I use alot of those words. I had to explain ‘anachronistic’ to my coworkers bt giving the example of antimacassar , which led to truncheon, which led to trepanation. . And they look at me funny when I didn’t know Ozzie Osborne was the lead singer to Black Sabbath.
Oh, and knobs. I regularly can be found knob twiddling, so to speak.
Get back here Skald 
Sorry, but that’s not a verb and is unfortunately used quite frequently.
I’ve used the word “Twiddling” in other contexts, notably “twiddling with [some piece of technology]” and “twiddling the controls/dials”. It has a marvellous Edwardian air of jiggery-pokery and earnest boffinism to it, I find.
Then again, my colleagues have also asked on more than one occasion if I have access to a dictionary that wasn’t written at the same time as the Jeeves & Wooster stories, so I’m probably not a representative sample for the purposes of the OP, I’m afraid.
Does anyone still use the verb “betrothe”?