Capricious

Meaning: You probably already know the meaning of this word, but Cotgrave defines caprice as:

A humor, caprichio, giddie thought, fantasticall conceit; a suddaine will, desire, or purpose to doe a thing, for which one hath no (apparent) reason.

I rather like the cynicism of that (apparent). Caprichio was a synonym for caprice, but now survives only in music.

Usefulness: 1 (Although if you wish to substitute giddie thought from now on, I won't blame you.)

Logofascination: 1 (It's a goat word! Probably! In fact, according to etymonline it's either from the Latin for wild goat, or a word about people with curly hair, and how can you not love that? I had a lightbulb moment when someone at work used it this week, and unlike a lot of etymological lightbulbs, this one was correct! Probably! Goats have a suprisingly strong influence on etymology, so I'm erring in their favour.)

Triboulet

Meaning: Cotgrave:  

a slovenlie fellow, one that usually weares his hose ungartered, and shooes untyed; also, the name of a famous foole belonging to King Francis the first; and thence, any fop, cokes, ridiculous ninniehammer, or laughing-stocke.

Triboulet was indeed a fool to Francis I, not to mention Louis XII. As well as this derogatory French term, he was apparently an inspiration for Rigoletto .

Usefulness: 1 (of myself, mainly, although technically I think that would make me a triboulette)

Logofascination: 2 (I admit to cheating here, and picking up a French word rarely used in English - wiktionary lists it, but not the OED.)