I know there's been an earlier question What is the meaning and usage of the word “beknownst”?. But nothing there satisfies my curiosity about that extra -st at the end.
I might have supposed the "extended" version to be dated / formal / literary, but I just heard it on the UK Channel 4 news. C4 newsreaders are normally far more relaxed about diction than those on other major channels (with glottal stops and dropped aitches aplenty).
So, what's the situation as regards actual usage? Is there a UK/US difference? And how does that extra -st come about in the first place? It doesn't seem connected to the archaic the second-person singular (as in Macbeth's "Thou com’st to use thy tongue; thy story quickly.").