Ah ok, I now understand what you were getting at @RichardBuck .
Arna' is short for arnaf.
In the context of discussing thirst, you can definitely say mae syched arna' i , which would literally translate as I have a thirst on me/upon me. This is commonly south Wales dialect. You will also hear people say mae annwyd arna' i for I have a cold and so on.
In the north we say mae gen i for I have. So I'm thirsty would be mae gen i syched, which literally means I have a thirst.
In some parts of the north you will also hear dw i efo syched. Efo meaning with, so dw i efo syched means I am with a thirst.
Whichever of the above forms you decide to use to inform others of your thirst/cold/cough/fear (and so on) you will always be perfectly understood.
Now just to confuse you further, you can say mae syched arna' i and mae arna' i syched. Or when discussing fear, mae arna' i ofn and mae ofn arna' i are both correct.... and so on.
When discussing things you owe, we also use arnaf/arna'. For example...
I owe you money - Mae arna' i arian i ti
I owe her an apology - Mae arna' i ymddiheuriad iddi hi
He owes me y favour - Mae arna' fo gymwynas/ffafr i mi.
I owe you for the petrol - Mae arna' i [arian] i ti am y petrol
I hope this helps!