Oh yes, I know it's Narsil!! Arthur just heard "Nostril" because he is not the sharpest blade in the drawer.

"Narsil, not nostril." Merlin/Olorin/Gandalf repeats impatiently. Perhaps Elrond was right that the fire had gone out of the line because there's not much burning in Arthur. "N-A-R-S-I-L."