Relative pronoun definition: He "what" runs

I would say the rule is to use "what" instead of "who" or "that" if you want to sound illiterate or uneducated!

What makes you associate this "What"-use with being illiterate and uneducated? I thought it was an older way of speaking, like saying "thee"?!

JTT: If I'm not mistaken, it is an older form of English. It's funny and more than a bit ironic that the language that is often made most fun of [hill country] more closely replicates older forms of English.

Right, I had deduced that as well, I like reading about old nuggets like "pray tell" and "fair teeming" and such, which is why this "what" thing caught my eye.
My generation of swedish youths is so used to modern english-american lingo that quaint, old fashioned expressions and colloquial figures of speech shine like pearls when we happen upon them.

It's not like I would use this expression any chance I get, that would sound pretentious as all hell, but I thought it would be interesting to see if there was any backstory to be found about the manner of this pronoun, like during which period it was most used and by which part of the english-speaking population.

Do you know how hard it is to try and hunt down linguistic web articles about aged relative pronouns on Google? It's a lot of work, I tell you.

Only a guess: but if we take 'what' to mean 'that which', can we call the 'older/hill people' use a kind of restatement? e.g.

1. The play what I wrote =>
2. The play that which I wrote.

'What' seems an unjustly maligned little word. In BrE, for instance, 'which' seems to be regarded as somehow more upmarket; so you might hear e.g.

3. 'I don't know which play you mean',

instead of

4. 'I don't know what play you mean'.

Or, monstrously, e.g.

5. 'I don't know to which play you're referring',

instead of

6. 'I don't know what play you're referring to'.

MrP