Several of the articles I read today on feudalism (see the much more important article below) mention that Latin feudum became French fief.  OK, but huh

Larousse tells me that the Chanson de Roland (1080) has feu, fiet.  Fieu was also seen.  Fine.  Then fief in the 13C with an f by analogy with bief, juif, soif

OK, bief ‘canal’ is from bedum, though it first appears as biez; juif ‘Jew’ is a back-formation from feminine juive from jūdaea; soif ‘thirst’ is 12C (earlier sei, soi) from sitis.  If analogy is happening, it’s not clear what the analogy is to; this is a pretty miscellaneous set.  (If you’re thinking sound change– no; intervocalic t or d normally just disappears.)

 But soif has a note: “f due to the false analogy with words of the type buef, bœuf, pl. bues, or of the type nois (nom.) / noif (acc.), from nix; the form in –f took over because it avoided homonyms.”

Off to bœuf ‘bull, beef’well, finally we have a word with a right to its f, from Latin bovemNeif/noif ‘snow’ comes from nivem but got replaced by neige, a back-formation from neiger ‘to snow’, from nivicare.

So apparently, in the 12C or so, French found itself with a couple of words with a morphological alternation between s and f.  Plus, er, the word juif.  This still is not telling me why the perfectly good words biez, soi, fieu got –f added to them, both in writing and in speech.

Hmm, French Wikipedia suggests that Larousse is full of it, or perhaps fullf of itf.  It derives fief from fevum, possibly related to a Germanic word vieh ‘livestock’, possibly confused with fiscum ‘royal domain’.  A French site, CNRTL, is close to this, but gives the source as fehu, related to Dutch vee ‘cattle’.  Fine, except for the -f again.  CNRTL offers that this derives from the verb fiever, itself created out of fieu.

The OED is of little use, except to point out that the –d– in feudum is hard to explain too.  It does reference fiever though.

Ha!  Another online source mentions a certain Palgrave who held that feudum derives from Greek emphyteusis.  Sure, Pal.

What a mess.  There’s a lesson for conlangers here too… if you want a naturalistic derivation, mess up your words till at least some derivations seem completely insane.  Then do it some more.