erstwhile
drawback
befoul
earnest
unforeseen
forbidding
hinder
uncouth
steadfast
setback
gladden
hold sway
right away
swine
"Erstwhile" is not a word I have seen used. The others are OK, except maybe for "gladden."
They all seem fairly common to me, I agree with Gil about "befoul".
not a teacher
I don't get what the OP is trying to express with "obsolete words". It depends on the context whether one can omit (or replace it with a less formal word) a certain words If the OP suggests that these words are obsolete in today's language I strongly disagree.
Last edited by Michael84; 12-Aug-2011 at 17:29. Reason: added ()
They're all used currently, so, regardless of frequency or context, I would say they're not obsolete.