I have never heard of 'ma's' - is it used in North Wales?
No, Ma's is used almost exclusively in south Wales.
What’s it short for?
I typed 'maes' here (which is what I had learnt in another course) but it was refused.
Yn ol ap geiriaduron mas yw out a maes yw field. Mae rywbeth yn fod yma? Rhaid sorto fe maes.
On this course we use the very common form ma's for the variant of allan. This reflects its pronunciation and also helps to distinguish it from two or three similar words.
On i'n trio dweud bod dylech chi derbyn 'mas' hefyd achos mae e pob man. Diolch ruis.