The Beer sisters, and their mother, were reading the book in the early 1930s, much closer in time to publication date than we are, and they couldn't work it out. It'll probably remain forever a mystery. Personally I'm still inclined to 'spy' - in the sense of "You're Lochiel's eyes and ears amongst my staff and that's going to curtail my freedom to do whatever I like," rather than the espionage/enemy agent sense. I suppose 'snitch' would be nearer the mark in present-day lingo. It's obviously a dreadful insult anyway.