Simpson: British Broadside Ballad and its Music' also has notes on the use of the tune, p570. He quotes a fragment from a medley (after 1610):
'Little pegg of Ramsay with the yellow hayre
& couldst thou greet if I were dead, marye would I feare'

This is just as likely to have been the song as mentioned by Sir Toby in Twelfth Night. There seem to have been several tunes/songs associated with Peg a Ramsey/Peggy of Ramsey in the 16th and 17th centuries. It could well have been a generic name for ladies of easy virtue. 'I'm goin ter peg ye afore 'e rams ye!'

Several of the milling ballads I have are real scots but none of these throw any light on the unexplained terms.

Barre Toelken in his excellent 'Morning Dew and Roses' (p122) goes into detail on German uses of the symbolism and indeed traces some of it back to the ancient world. 'In symbolic language, mill means the feminine member (mullos, from which mulier), and the man is the miller, and thus the satirist Petronius uses the term 'molere mulierum' for cohabitation......on which passage the Talmud comments under 'milling' is always to be understood the sin of cohabitation.