[Legate of Amon Lanc]

Originally Posted by jallanite

From The War of the Jewels (HOME 10), page 383:

In S. the word gûl (equivalent to Q ñóle) had less laudatory associations, being used mostly of secret knowledge especially such as possessed by artificers who made wonderful things; and the word became further darkened by its frequent use in the compound morgul ‘black arts’, applied to the delusory or perilous arts and knowledge derived from Morgoth.

This explanation suggests, but does not prove, that when Gandalf uses the term “Morgul-knife”, he is referring to the knife being a knife of black magic rather than to it being made in Minas Morgul. Of course Gandalf might have meant both at once.

Of course he meant both at once, but I think only because the place name "Morgul" is derived from the word for sorcery.

Also, a sort of "technical" note, even though the HOME offers many sources and notes on background of many things, the title "Morgul-knife", as we speak of it, is used within the context of LotR, where basically the only use of the word "Morgul" is with capital "M" and it is a toponym, the name of either Imlad or Minas Morgul (and derived terms, like captain of Morgul-hosts etc.). It is pretty clear that when somebody is talking about for example "Lieutenant of Morgul", "Morgul-host" or "Morgul-road", he does not mean "Lieutenant of black sorcery" or "army of black sorcery" or even less "Road of black sorcery", but the captain of/army/road belonging to/leading to the particular place. There is no reason to think about the knives otherwise.

Of course the meaning of the word is "dark sorcery" (as one can look it up also e.g. in the short Elven dictionary in Silmarillion, no need to go as far as HOME), and of course upon hearing the name, a person who knows Elvish would get both the connections in his mind. But there is no reason for Gandalf to speak of "Morgul-knife" in front of Frodo, who, even though he knows Elvish, would probably be rather confused at hearing that. If "Morgul-knife" really meant just "knife of dark sorcery", Gandalf could say just that: "You were hit with a cursed blade," perhaps elaborating a bit on that. It would just seem a lot unlike Gandalf to use the Elvish word "morgul" so randomly (as it seems to me) instead of just saying "sorcery" (as a more easily understandable equivalent, if the situation was like you say it was), since Gandalf is not the type of person who would enjoy using complicated terms when there is no reason to use them. In other words, it is as if a doctor-Gandalf told recovered Frodo: "you are suffering from supracondylar fracture of distal humerus" instead of "you broke your arm". I believe real Gandalf would rather use the second formulation.