I’m starting from the premise that you just can’t say “unbeknownst” anymore, not without sounding at least a little arch and like you’re making fun of the people who use words like “unbeknownst” without meaning to sound a little arch and like they’re making fun of (I’m sorry, I have to call this sentence off because of the recursion error). Anyway, if you’re not willing to give me that premise we’re just not going to get anywhere.
The thing is that “unbeknownst” was a perfectly good word, usable for all sorts of conditions when the beknownsting of things was aptly described as un-, and now it isn’t. At some point the comical uses of the word so overwhelmed the serious uses that the word had to be given up as part of the usable non-humorous vocabulary.
So that means there’s someone out there who was the last person to use “unbeknownst” without meaning it archly, and without expecting the audience to hear it a little archly, and given that arch uses of the word had to be on the rise then I wonder: was this last-serious-use of the word something the writer knew was going on, or was it unbeknownst to her? But if it was beknownst to her, doesn’t that keep it from being a legitimate use? Even a little bitty bit? Or did she refuse to think about that lest she lose the spot of last-legitimate-user to whoever used it just before she did?
To sum up, worrying about this nonsense is why I got like two hours of sleep last night.