I know there are plenty of words that use the -st ending: wouldst, whilst, unbeknownst, etc. but I’m not really sure what it means to add an -st suffix to a word. What does it mean to add the suffix? How can I tell what words can take the -st suffix? Are there any modern words that can take the suffix and not have people look at me in a strange way?
Your -st endings are two different grammatical animals. In wouldst it is the standard verb ending for the archaic second person singular familiar thou:
Cleopatra. O, I would thou didst,
So half my Egypt were submerged and made
A cistern for scaled snakes! Go, get thee hence:
Hadst thou Narcissus in thy face, to me
Thou wouldst appear most ugly. He is married? — Antony and Cleopatra 2.5.1174–8.
Unbeknownst, however, is another creature entirely, even among its peers.
Emerging in both British and North American print sources in the 1830’s, unbeknownst was originally a colloquialism coined on the pattern of much older words such as unawares (1530s) or always (early 13th c.), adding the etymologically intrusive final t of amongst, whilst, betwixt, etc. The s is a remnant of the genitive case, often used in Old and Middle English to form adverbs from nouns or adjectives.
I think that ‘ere Ingian must have been the devil, or how could he come so sudden and unbeknownst upon me, … John Richardson, Wacousta, or, The Prophecy: A Tale of the Canadas, London, 1832.
“I remember my respectable first-floor, Monsieur Boncoeur, bringing home a piping bullfinch last year he’d bought on the Boulevards, whose red breast washed off the first showery day, all as one as Ma’mselle Isoline’s rouge after a flood of tears in a melodrame! The poor dear gentleman had half a mind to have up the seller of the impositious bird before the commissary of the district; only, as he’d paid for it with an old coat unbeknownst to his valet, and an old coat not being lawful coin of the realm, there was a doubt in his mind about his power of bringing the vagabond to justice.” — Toby Allspy (pseud.), “Adventures in Paris: The Five Floors,” Bentley’s Miscellany 2 (1837), 501.
“No matter,” added Lynx authoritatively ; “getting into another man’s barrel unbeknownst to him in the night-time, is burglary.” — Joseph C. Neal, Charcoal Sketches; or, Scenes in a Metropolis, Philadelphia, 1839.
The word also appears in an 1875 Sussex dialect dictionary, suggesting the usage is older than its first appearance in print:
“All I can say is, if he comes here, it’s quite unbeknownst to me.” — William Douglas Parish, A Dictionary of the Sussex Dialect and Collection of Provincialisms, Lewes, 1875, 127
Even so, the word is a likely candidate for the last coinage in English using the adverbial genitive.
The adverbial genitive, though no longer productive, is a feature especially common to West Germanic languages. It’s hiding in plain sight in many common English words: once, twice, sideways, backwards, forwards and the alternatives amongst, whilst more common to British usage.
This means that you can no longer form words using the adverbial genitive -s, and unless you’re in a Shakespearean play, you won’t be using the thou form of English verbs anytime soon.