Hee hee I'm glad I've provoked you into spending half an hour a-googling Steve!

For what it's worth, I reckon 'lass' is a term used all over the north of England as well as Scotland.
Lassie would be more of an endearment or sentimental use of the term.

I used to weep buckets into my Uni (twee?) scarf hearing that folk song, "Will ye go lassie go? And we'll all go together..." I don't think it was about a chap trying to train his sheepdog.

But more than one lad rolling his lass in the new-mown hay would surely be termed an orgy?