Where did "unbeknownst" come from? And why has it become popular in the media in recent times? Is it some sort of superlative from "unbeknown"? But what is the advantage of that over "unknown", anyway?
The OED only notes the words from mid to late C19 so they are not, as one might say, part of the basic Anglo-Saxon vocabulary. "Beknow" is said to be "Long arch. and dial." and to mean "be or become acquainted with; recognise, acknowledge, know." I wonder what makes "unbeknown" seem superior to "unknown"?
Perhaps I can see some usefulness in a sentence like "Unbeknown to me, the package had already been delivered." "Unknown to me, the package" might seem some sort of dangling participle. It wasn't the package that was or wasn't known but the fact of delivery. Maybe the use of the "unbeknown" is some sort of signal that we are thinking of awareness of facts, rather than knowledge or acquaintance with things. But to object to "unknown to me" would be to get pretty pedantic, wouldn't it?
And then what further advantage comes with the -st? Suggestions and comments, please.