Journal of Other Forum Analysis

[NekoFever]

in the books it wasn't because she was a woman, I think it was because merry stabbed him in the ankle with a dagger they had extremely luckily picked up in a tomb which happened to be dead people he had been oppressing as their overlord in life, who hade made this superweapon dagger they never got the chance to use on him 

There was a prophecy that "not by the hand of man will he fall" in the books, but the way Tolkien writes it can apply to Merry as much as Eowyn.

And I don't think the dagger had any specific strength against the Witch-king, just general spells against Sauron and his servants from a past war.

But anyway,