What does "in one fell moment" mean?

Question

His heart pounded firecely for revenge. It mattered not that he would die as long as he could wash clean his pain and sorrow in one fell moment. All he had to do was break cover.

Answer 1

I've heard of "one fell swoop", but never a fell moment. Since the "fell" in "fell swoop" means "all at once," I would assume it means that here too, but it doesn't make much sense since a moment is actually a measured period of time!

Answer 2

i think it means fast. Just like in one fell swoop which implies all at once. All at once is fast.