I've used "beknownst" and "unbeknownst" a couple times but I never really bothered to look it up until now. But it's not in most of the online dictionaries websites I frequent. I'm under the impression that this word is old school; if so, is it still appropriate to use? Or am I better off using some other word that means the same thing?
Technically there is no such word as 'beknownst' other than as a back-formation of 'unbeknownst' (unbeknown exists and in fact pre-dates unbeknownst. Beknown also exists). It is rather old-fashioned which is probably why some online dictionaries don't list it.
They have essentially been superceded by 'known' and 'unknown', which have equivalent meanings. Nevertheless it is not uncommon to hear:
Unbeknownst to me, my wife had already bought the same present.
Or the like. It is perfectly appropriate to use.
beknownst is an archaic word, as it is beknown; the meaning of beknown is known.
The word that is used in Modern English is unbeknown (or unbeknownst), which has its origin from beknown.
unbeknown |ˌənbəˈnoʊn| (also unbeknownst)
adjective [ predic. ] (unbeknown to)
without the knowledge of someone: unbeknown to me, she made some inquiries.
ORIGIN middle 17th century: from un- (not) + archaic beknown (known).
Unbeknownst is strictly an adverb may not be benoted as much as unknown since unknown is a noun and an adjective.
However, it is hardly "archaic" ... I eathly found samples of being in magazines and books in the last decade.
The beverage industry has long supported groups such as Keep America Beautiful (the group famously known for its " crying Indian " ads) that emphasize individual responsibility for Utter collection but which, unbeknownst to most consumers, work behind the scenes to oppose and defeat bottle recycling bills. … E: The Environmental Magazine, 2011
And it is benoted much more than unbeknown: Ngram (Sorry, I couldn't get it to load here.)