Explanation of "must needs"

Needs is an old-fashioned or even archaic adverb in modern English. It comes from the noun need and the Germanic masculine/neuter genitive ending -s, which at some point in time came to be used in older English and other Germanic languages to form adverbs. From the Oxford English Dictionary:

In once, twice, thrice, hence, since, etc., the suffix is written differently. In against, alongst, amongst, amidst, and the dialectal onst (see once), the original -es, -s has become -st.

This suffix is still frequent and semi-productive with other stems in Dutch: onverwachts "unexpectedly", daags na "a day after", etc. I believe German has similar examples. English always, sideways etc. contain the same suffix.

In Old English or earlier, before the emergence of the adverbial suffix -s, there existed several masculine and neuter words in the genitive case—which ended on -(e)s—that were used adverbially: dæges "by day", nédes "needs", þances "voluntarily". By analogy, this masculine/neuter genitive ending was extended to make adverbs out of feminine nouns as well, as in nihtes "by night". From there it became a general adverbial suffix. (Examples were taken from the OED.)

I'm not so sure it "wound up" an adverb/adjective. Chaucer wrote A man must needs love mauger his head (where I'm guessing mauger relates to French malgré = in spite of)

Thanks to @pavium for this link - which deserves more upvotes than my answer!

The usage is generally archaic/formal/academic. It probably survives partly because it echoes the equally archaic needs must [when the Devil drives].

It's not a "plural verb" (nor "second person singular verb", nor "plural noun"). I think it's always been an adverb meaning "necessarily". I'm not aware of any other words that happen to be both noun and verb and have an [archaic] adverbial form that's the same apart from ending is 's', but even if there were, I doubt you could call it a meaningfully productive 'pattern'.

Semantically, needs functions as an intensifier to the preceding must. It doesn't really need to be present - it's just adding emphasis, with perhaps a touch of gravitas from the archaic overtones.

This question was discussed here:

http://www.randomhouse.com/wotd/index.pperl?date=20010817

While the reference classifies needs as an adverb, I think it owes more to [the imagined requirements of] conjugation than number for the 's' at the end. (as in 'I need', 'He needs', etc)

It's more likely a writer affecting a older style of writing, in line with the subject of corporal punishment.