Why (and for whom) does "unbeknown" become "unbeknownst"

Solution 1:

The OED says they don’t know where the -st came from.

unbeˈknownst, a. or adv. Orig. colloq. and dial. Also unbeknowns, etc.

Etymology: f. prec. The analogy on which the -s or -st has been added is not clear: cf. the earlier unknownst.

= unbeknown (def#2). Also, = unbeknown ppl. a. (def#1). Now of much wider currency than in the 19th. cent.

I emboldened the last sentence because I have only ever heard unbeknownst myself, never this unbeknown thing. Citations for unbeknownst go back only to the 19^th century, but for unbeknown to the 17^th.

Here are the first few for unbeknownst:

• 1848 Mrs. Gaskell Let. 11 Nov. (1966) 61 — You don’t see me, but I often am sitting in the rocking-chair unbeknownst to you.
• 1854 Huxley in L. Huxley Life & Lett. (1910) I. 111, — I hate doing anything of the kind ‘unbeknownst’ to people.

And here are the first two citations for the other one:

• 1636 T. Goodwin Return of Prayers iv. 75 — To sympathize with another in praying for such a thing unbeknowne one to another.
• 1836-7 Dickens Sk. Boz, Seven Dials, — If my ’usband had treated her with a drain..unbeknown to me, I’d tear her precious eyes out.