(Shameless self-promotion: if you like this subject, consider !linguistics@mander.xyz )

It’s being used as an adversative conjunction, connecting a phrase (usually a clause) with whatever precedes it, in a way that highlights that the precedent would incorrectly imply something.

Here are some examples showing it:

1. “That ‘tho’ is like a ‘but’. But it’s used at the end of the sentence.”
2. “That ‘tho’ is like a ‘but’. It’s used at the end of the sentence tho.”
3. “That ‘tho’ is like a ‘but’. It’s used at the end of the sentence.”

Examples #1 and #2 are pretty much equivalent: the first sentence introduces an information (that “tho” is like a “but”), that information implies something incorrect (if “tho” is like a “but”, it would go at the start of the preceding sentence, right?), and the second sentence contradicts said implication (nope - it goes at the end). With the “but” or the “tho”, that contradiction is explicit.

Now look at #3 - it sounds like [incorrectly] saying that “but” goes at the end of the sentence, unlike #1 or #2.

The idea of a conjunction going after the elements being “conjoined” might sound a bit weird, but it’s worth noting that it’s nothing new. Latin for example used -que (additive conjunction; “and”) this way, it went after everything that you were joining. (Classical examples: “arma uirumque cano” [I sing the arms and men] and “Senatus Populusque Romanus” [Roman Senate and People].

---

Now, on why it’s being used this way: there’s the spelling and the increased usage.

“Tho” as a short form for “though” is actually old as fuck, with Merrian-Webster claiming that it was already uncommonly used in the 18xx. It can be seen as a short form that became more socially accepted nowadays, at least in informal writing. And in special, this sort of “grammatical word” (conjunctions, articles, adpositions, copula verb etc.) tends to be rather small, as you’ll use it all the time.

And the usage of “though” as an adversative conjunction is even older, being attested at the 12th century. Probably way older given that cognates in other Germanic languages also have the adversative meaning, even the Old Norse descendants like Icelandic.

I’m not sure on what I’m going to say, but I think that the increased modern usage is the result of some changes on how people interpret “but”. A lot of people have been treating “but” as if it contradicted completely the preceding discourse, like:

• Alice: “I wanted a banana pie. Not an apple pie.”
• Bob: “Why do you hate apples?”
• Alice: “I like apples, but I like bananas better.”
• Bob, who stopped hearing at the “but”: “THAN U DUN LIEK APPLES!”

That probably led to increased usage of “though” because it’s used after whatever you said the relevant piece of info.