Not for the first time I've been caught out talking through my hat and, of course, you're correct although, sadly (to take you out of context slightly), Welsh speaking is still not 'really common all over south Wales'). What I should've said (in my ignorance) is that the form 'ma's' (as opposed to 'allan') was not previously familiar to me. But that's no excuse and, on consulting Alun Rhys Cownie's excellent Dictionary of Welsh/English idioms, I deservedly stand condemned of being 'ma's ohoni' or 'ma's o drefn' or even 'ma's o reswm'. Or perhaps I could plead to being 'ma's o hwyl' at the time. Anyway (genuinely), diolch: I've learned something useful.