I thought this would be a nice night cap but damn! It's a bit much for this late in the evening.
I don't know what the origin of the Rusty Nail is. It does not appear in any of the books I have from the 40's and 50's. It does make an appearance in the 1971 Playboy's Host & Bar Book, which I highly recommend that you pick up if for no other reason than the fabulous photography. (Sorry, no nudes, just great 70's food, drink, and fashion photography). In that book the drink is made of Scotch and Drambuie in equal measure, though DDG calls for a ratio of 2:1.
In Gary Regan's Joy of Mixology, he calls for a ratio 5:1, but explains that "this is a good recipe for experimentation, since by using different bottlings of scotch and altering the rations of base spirit to liqueur, the cocktailian bartender can learn much about ingredients."
This is a lesson I learned the hard way tonight, when I picked up the Highland Park which Tipsy opened by mistake last week. At 53.8%abv it is a bit more stout than some other Scotches I could have chosen. But I've made this bed and now must lay in it...