Billibodring

Meaning: Williams suggests that this means 'phallic jousting', hypothesising that the billi element is related to billy-goats (notoriously lustful), and the bod a shortened bourd, which has two meanings: jesting or banter, or playful jousting.* As an Urquhartian original, meaning and etymology is speculative; you could make up your own.

Usefulness: 1 (metaphorically - think of all those situations where guys are attempting to out-bloke each other. "If you two are done with the billibodring, can we get the meat off the barbecue?" "The meeting was pointless; the suits spent the entire time billibodring." "Dinner was awful, but the guys were billibodring about their cellars, so we had some incredible wine.")

Logofascination: 1

Nebrundiated

Meaning: One of Sir Thomas' coinages, it's clear from the context that it indicates sexual activity of some kind. Gordon Williams suggests 'energetic pricking'; and helpfully goes on to speculate on etymology:

Neb may return us to the bird’s bill, but Dict. Of the Older Scottish Tongue notes it as ‘the tip of one of the protruding parts of a person’s body’. Rund = to make a grinding noise.

Usefulness: 2 (Since it's a blank slate, we can practise our word-building: one's life may be nebrundiationless, denebrundiated, anebrundiated, omninebrundiated, mononebrundiated, binebrundiated... and so on.) 

Logofascination: 1 (If Williams is correct, Sir Thomas is sneaking Scottish dialect in here; again, though, in the mouth of an outsider in the text, Herr Trippa.* There's a thesis of some sort in there, I'm sure: "Scots Dialect and Sir Thomas Urquhart's Conception of Otherness in Rabelais.")

Flay the fox

Meaning: Cotgrave gives us a definition and the only etymological speculation I can find online:

"Escorcher le regnard.  To spue, cast, vomit; (especially upon excessive drinking;) either because in spuing one makes a noyse like a Fox that barkes; or (as in Escorcher) because the flaying of so unsavorie a beast will make any man spue." (Current French would be écorcher le renard.)

Usefulness: 3 (depending on your lifestyle. It should be noted that flay the fox sounds suspicious even if you don't know its meaning, so the French version could be useful if you need to describe a particularly good or bad weekend in front of the boss.)

Logofascination: 2 (mainly because it's slightly mysterious slang; I'm not quite sure what it is about old terms for bodily functions, but they're popular.  Jonathon Green has written an almost poetic list of synonyms for vomit, and points out that Australians have originated or are the main users of 38% of these terms. Having recently translated the lyrics of Land Down Under for some Americans, I can't comment.)