“The dagger, which is a shortened sword, or a lengthened poignard, was called in the 14th century Misericord, because, thrust at the throat of an overthrown adversary, it forced him to ejaculate its name. In the 16th century, the dagger no longer goes to war; it becomes a duelling arm—terrible and traitorous; now parrying the rapier-thrust, anon insidiously insinuating itself into the openings of fence, to deal the adversary a ‘coup de Jarnac’: sometimes the dagger, naked and smooth to the eye, hid within itself a trident: two little lateral blades, incorporated with the central one, bifurcated under pressure of a spring hidden in the shoulder. ‘Twas a viper protruding its triple tongue of venom.”
~ Paul de Saint-Victor, Anciens et Modernes (1886)
A “coup de Jarnac” is basically a sneak attack. At first it meant “a devastating, skillful and unexpected strike”, emphasis on skill. It originated from a trial by combat in 1547, where Guy Chabot, baron de Jarnac, defeated his opponent by striking him behind the knee — he basically hamstrung him. The move was new and unexpected as far as the audience (and his opponent) were concerned, though Jarnac didn’t invent it, he learnt it from an Italian swordsman that he’d hired before the duel. It wasn’t considered dishonourable at the time, only especially adroit and effective. No one disputed the duel’s result, and the king declared Jarnac the victor.
But a few centuries later, the meaning of the expression coup de Jarnac shifted from “skillful blow” to “traitorous”, precisely because it was unexpected. It didn’t have to be about combat, it was also used metaphorically for an act that ruins someone or destroys his fortune, sort of like the English backstab.
From there, jarnac came to mean “dagger”, and that’s where Paul de Saint-Victor picked it up, and went on to describe daggers as terrible, traitorous, insidious, venomous, etc. This Rogue, for one, takes it as a compliment. :p
P.S. The English translation of Paul Saint-Victor is from the prologue of Worke for Cutlers: A Merry Dialogue betweene Sword, Rapier and Dagger (a 1615 play, edited by Albert Forbes Sieveking, Cambridge University Press, 1901). I changed “glaive” to “sword” for clarity. (The translator would expect his 1901 audience to recognise “glaive” as a French and therefore poetic word for sword, but for modern audiences the glaive is a polearm, and it would be terribly confusing.)