Thank you, Guest. Still don't know quite what she meant by it, but that certainly seems to be the origin. Wonder why it should suddenly have caught on about - what[?] - 10? - years ago,, when, judging by her d.o.b, it must have been ongoing for a while by then; or whether the 'big' is descriptive or intensificatory. But certainly a start — many thanks again.

Any other, equally daft, ones? In the Forest of Dean, where my wife cane from, btw, 'mard' means 'spoilt' [presumably = marred], so 'mard-arse' would mean spoilt brat.