Copesmate

Meaning: It eventually meant an associate, friend, partner, but the OED defines its oldest sense as "A person with whom one copes or contends; an adversary, antagonist."

Usefulness: 1 (Someone you cope or contend with? This word is ripe for resurrection regarding office companions, housemates, and - depending on your family - siblings. "A new copesmate joined our team today; she bought muffins, so we think we're going to like her.")

Logofascination: 1 (This makes more sense when you realise that cope originally meant "to come to blows with". Like cope, copesmate has evolved from that antagonistic origin, with a diversion into meaning adulterous lover, paramour and/or spouse.)

In the wild: Shakespeare uses it in The Rape of Lucrece: 

'Mis-shapen Time, copesmate of ugly Night,
Swift subtle post, carrier of grisly care,
Eater of youth, false slave to false delight,
Base watch of woes, sin's pack-horse, virtue's snare;
Thou nursest all and murder'st all that are:
O, hear me then, injurious, shifting Time!

Degrees: 1 (Sir Thomas' G&P is cited by the OED under the "paramour" sense, although he also used it to mean spouse and adulterous lover at different points.)

Connections: n/a

Which is used in: G&P, Book the Third, XXVIII:  How Friar John comforteth Panurge in the doubtful matter of cuckoldry. Friar John tells Panurge the story of an old jeweller who marries a young wife:

Notwithstanding all this, he found her always more and more inclined to the reception of her neighbour copes-mates, that day by day his jealousy increased.

It does not end well.