I have seen multiple suggestions to what his name might main, but I see it as this.
Nah - Fury, as we have all established.
Fahl - for (your), on behalf of (your)
Aar - Loyalty
This would have two interpretations:
The first being that he shows fury to others, perhaps to protect his own people, because they were loyal to him.
The second, which I deem more likely, seeing as dragon names usually show characteristics of the dragons bearing them, is that Nahfahlaar shows fury out of loyalty. As soon as he is loyal to someone, he stays loyal and will show fury to anyone threatening this bond.
This someone he might have bonded with could well be Talos. Perhaps he even changed his name to this, because he was so loyal to him.
Just my two cents.