What does the -st word ending mean and is it used in any modern vocabulary?

What does the -st word ending mean and is it used in any modern vocabulary?

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 fix and not have people look at me in a strange way?