That ubiquitous '60's folk strum(You practically see Dylan appear in the air when you play it) , that goes,"Bum-Bum-dadadada Bum-Bum-dadadada", what is it called, and where did it come from?

I suppose that I should know this--I have showed so many people how to play it over the years, and low and hehold, someone finally asked me about it, and all I could do is make a smart remark ("That strum and three chords would get you into any hoot....") Anybody??