My wild guess (until someone else joins in): How about -
“( Mae’n) beth da (bod) ti’n yma” for Its a good thing that you’re here? Ok, I cheated by changing ti’n da into da 'ti’n.
The only other thing that comes to mind is 'da for gyda (with), but that still sounds a bit weird.
I’ll be looking out for the real answer 
Snap! What are you good here- what are you with here- it must be slang! If only someone had been on the set lately!, @philipnewton!!! @aran can you help?
Its not “beth? wyt ti’n dal yma?” is it? As in"what? are you still here?"
Not really short for anything - but it’s a common usage, which is a bit like the English ‘good for something’ or ‘good for nothing’ - so literally, ‘what are you good for here?’ - kind of odd that it doesn’t get mutated, either, but there we are…
Yn yr her rhif tri, de lefel dau… dwim yn siŵr beth @iestyn yw ddweud am 'We/You don’t '. A yw hwn yn gywir;
In challenge number three in the second level for south, I’m not sure what @iestyn is saying for 'We/ You don’t '. Is this correct;
‘sochi’n gweld’ a/and ‘soni’n gweld’
Mae’n ddrwg 'da fi @iestyn a cat! Dwi’n trio yn galed iawn i glywed a ddeal chi! 
This may help
O! Diolch i chi!
Diolch Pete, and da iawn cwningen!
The negative “So… person… yn” instead of “dydy [+ending]… person… ddim yn” is very common in the south, so:
dydych chi ddim yn > So chi’n
dyde e ddim yn > so fe’n
dydyn ni ddim yn > so ni’n
etc
Ah! Thanks!
I had wondered whether this was “good” but had dismissed that possibility due to the lack of mutation…
But that makes sense.
From another Facebook status, this time a young person from Aberystwyth:
“Ffili credu…” with the poster’s bilingual explanantion “Can’t believe…”.
My guess is that it’s local for ffaelu : fail (to).
Is this in common use? If so would Ffaelu only be used where “fail to/failed to” would be ok in English.
I think David on Rownd a Rownd says this occasionally as well – I think it’s usually subtitled ffaelu but sounds like ffili when he says it.
I think the most recent occasion was “I didn’t manage to / didn’t get around to (tell Dani I was breaking up with here)”, which fits better to “fail to” than to e.g. “couldn’t” (gallu) or “didn’t” (peidio).
Yes, very common usage.
Ffili is pretty much the southern version of methu.
What is Bechod! (or something like that), and what does it mean?
I hear it most often from Dani on RaR, often in a context a bit like “the poor thing!” (like Creadur!) but sometimes not, just after hearing something negative perhaps.
The Ap Geiriaduron says “Bechod!“ = “Shame!“. Seems to fit nicely with the connotation you mentioned.
Literally, ‘pechod’ is ‘sin’, but it’s used as Hendrik says in the sense of ‘what a pity’…
Bechod and bendith are two of the things that we’ve taken to using as a family to an extent that even the youngest no longer ask “er what does that mean” like they do with so many other things I say!
Bit by bit Welsh is becoming part of the everyday for us.
What’s bendith, then? Literally, “a blessing”, I presume, but how is it used?
Often when someone sneezes…
I’ve also heard it as a rather elaborate, slightly over-the-top way of thanking someone, but not very often.
It’s after a sneeze! (or should that be tisian!)
Right coffee break over - back to work…