UnbeknownstThe word unbeknownst is almost always used in the form of a phrase preceding a statement: "Unbeknownst to person, statement."
RegisterEver since I first encountered this word, I have assumed that it is always used in a jocular, humorous, or self-mocking way (as highfalutin, speechify, and snazzy are). Certainly unbeknownst sounds more deliberately old-fashioned than unknown, with its verb prefix be- and its archaic second-person singular verb ending -st (as in "thou hast").
But having seen it recently used in a non-ironic, non-jocular way, I looked up this word for the first time and found, to my surprise, that it is not marked as anything other than standard English: not informal, not humorous, not jocular. And according to one dictionary, the -st ending is not a "thou" verb ending but rather the same -st ending that is seen in amongst (or midst).
Moreover, unbeknownst is listed as a variant of unbeknown (a word which I have never seen in print).
Part of speechWhat part of speech is unbeknownst? All the English-language dictionaries I looked in say that unbeknown and its variant unbeknownst are adjectives. But the phrase "unbeknownst to person" is always used to modify the statement that constitutes the main clause of the sentence. So maybe unbeknownst to should be construed as a prepositional phrase that modifies a whole sentence (as an adverb does). Indeed, the PONS English-German dictionary labels unbeknown(st) as an adverb, which in my opinion does seem more accurate than calling it an adjective. The word unknown is certainly an adjective, and a sentence like "Unknown to anyone, he led a double life." seems vaguely ungrammatical (what noun does unknown modify?). But "Unbeknownst to anyone, he led a double life." seems perfectly grammatical. So it looks like the practical function of the prefix be- and suffix -st is to indicate that the word is being used as an adverb rather than as an adjective.
(November 16, 2003)