Could there be any similarity to the Irish "Planxty", which similarly defies dictionary definition (at least in my Compact Oxford Dictionary)!

For want of any better definition, both "maggot" and "planxty" would appear to designate a tune composed in honour of or at the behest of a patron.

That's what I have always thought, but I am sure someone here will disillusion me or correct me if I am wrong.

Alan