system
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human
[LOTR] What does “no man can kill me” actually mean? Does it refer only to adult human males?

Technically, Merry is a Hobbit and Eowyn is an adult human female. Neither fit that definition of “man.” But did the prophecy use a looser definition, where any male would qualify, or any human?
gpt
I feel there are a few misconceptions regarding the Witch King's battle with Eowyn.  


First, he technically does not say "No man can kill me" but rather **“Hinder me? Thou fool. No living man may hinder me!"** as a response to Eowyn's demands that he cease his attempts to desecrate the body of King Theoden. The Witch King is the chief of the Nazghul and a powerful wraith who straddles the world between the living and the dead.   


He is essentially saying "I am the chief of a powerful group of wraiths, kept alive through dark powers beyond mortal understanding. Who the hell are YOU, peasant?"  


As for the prophecy regarding his defeat, Glorfindel says:  


**"Do not pursue him, he will not return to this land. Far off yet is his doom, and not by the hand of man will he fall."**  


Our favorite balrog-slaying elf isn't saying that the Witch King is magically impervious to male warriors, but rather he is foreseeing the Nazgul's downfal in the distant future. He is delivering this prophecy to King Eärnur who tried to destroy the fiend at the Batte of Fornost but failed due to the Witch King's dark aura frightening his horse.   


Basically Glorfindel is telling King Eärnur he is in over his head, not because he's male but because his warrior's pride is no match for the Witch King's dark magic.   


I would further argue that the Witch King's arrogance against mortal men (whom he has far surpassed through the power of his ring) is what sets him up to fail.  He's overlooked the Daggers of the Westernese, forged by men long dead. Daggers which were deliberately enchanted to do harm to his kind.   


The reason Merry and Eowyn are able to triumph in their fight with the Witch King is because Merry's dagger is able to nullify the Witch King's dark magic. Eowyn, empowered by her desire to avenge her fallen Uncle, is able to soldier through the lingering Black Breath to deliver the coup de grace. Even weakened, the Witch King was still an unholy abomination of dark magic.   


So in short, the Witch King isn't magically protected from human males but rather is too powerful for the average man to slay on account of his dark magic. His death at Eowyn's hands is just delicious irony because, hey, technically a man DIDN'T kill him in the end.