From Hayden Carruth's memoir of James Laughlin, Beside the Shadblow Tree:
Incidentally, Jas was the one who taught me that in the succession of adverbial ordinals it is correct to say "first, secondly, thirdly," etc.
Carruth goes on to say: "Not one in five professional copy editors know it."
Secondly is striking because it is extinct. People do not say secondly as often as they do say -- cover your ears -- the word hopefully.
Some months ago, a woman speaking on her cell phone told the listener that she would be there momentarily. That one is merely rare, not quite extinct, thanks to electronic phone drones. Debatably, though, one could say that the phone drones have ruined the word, and so it's one of the walking dead.