Answer:
To you, rude would I never be, though I flag my tongue for all to see. What am I?
A single syllable do I claim black was my most famous name; Fetal to mortals here below thousands have I slain in a single blow.
Deep, deep, do they go. Spreading out as they go. Never needing any air. They are sometimes as fine as hair.
I go through a door but never go in and never come out.
I am an English word that is as long as I am valued. What am I?
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.