420

[BlueWillowPlate]

I think the get high aspect of 420 came long before the Hitler's birthday aspect.
When we used the term, none of us were referring to Adolf.
(This was in Denver back in the 70's, as opposed to Denver suburbs in the 90's.)

[Magnolia Bloom]

Well, I always heard that Florida was behind in the trends.... I've heard, er, read.... want to blow a doobie, want to get lizard-eyed, wanna get a head on? Now I hear, wanna go to the earlybird after your nap?! But that involves a buffet, not a blunt!

Oh, and another phrase- Are you wearing your green suit?

[Marodi]

Anyone that's in college or highschool knows what 420 is. Everyone. Also funny video on Youtube of a guy on Price is Right only wagering 420

[Willys]

420 is the flight number in the 1954 movie 'The High and the Mighty' , starring John Wayne and Robert Stack, as well as other notables. The movie had its moments including several great shots of Robert Stack's ass, though not bare.

That may not be the origin of the coined phrase, but certainly lends itself to cause.