The fullest background on this song can be had by getting The Steel Bonnets by George MacDonald Fraser - an immensely readable history of the Scottish/English border reivers from 1286-1603. "Ah ha!" you may say, "moss troopers didn't pop up on the border until about 1650." On reading the book, and matching it with the sentiment of the song, though, you might decide the author, as is common, has misapplied the term, and is really refering to the reivers of earlier times.