Obsolescent, then? (Not you personally, I hasten to add). "Arch" is one of those words that, for decades, has drifted around the periphery of my passive vocabulary, at a distance such that I've generally had to have a quick glance at the dictionary to make sure I understood it correctly when encountering it. However, never once have I used it actively. I feel that it has a literary quality that makes it an unlikely candidate for my own prosaic excursions into print (or the online equivalent). My thesaurus places it alongside pietistic (sic), sly, nudging and winking. I'm not sure how I would express myself if I wanted to say something along these lines.I'm not sure I agree with the Doc about this adjectival use being effectively obsolete. I use it myself
a. [Arising from prec. sense, in connection with wag, knave, rogue, hence with fellow, face, look, reply, etc.]