Who was the first to use Drop D tuning?

Help me out guitar enthusiasts.

Drop D tuning is nothing new, and it's been used longer than rock and roll has bee around. Many old time folk and country performers like Jimmy Driftwood, and Jack Elliot had used it. I'm sure it was used long before their time as well.
Other notable guitarists during the 50's-70's to used drop D tuning are;
Duane Eddy
Leo Kottke
Johnny Winter
Duane Allman
Eddie Cochran
Rory Gallagher
Alvin Lee
Link Wray
Dick Dale
Jorma Kaukonen
Leslie West
Jimmy Page - Listen to 'Moby Dick'. The whole tune is based on a single note riff that uses the lowered sixth string in drop D tuning.
So it's impossible to say who was actually first.

I would go out on a limb and say Eddie Van Halen but I can't be 100% sure

no one will know. there were probably older guitarists that were never very famous who experimented with it. besides, all drop d does is make power chords and riffing easier

It's been around for decades. It does much more than make power chords easy. There are some great songs that used drop and double drop.Dan Fogleberg used it quite often. Neil Young used double drop d in Cinnamon Girl and the Doobies used it in Black Water.