A reference to being very, very high.
Based on the internet video where a guy makes it abundantly obvious that he is extremely stoned through his astonishment of viewing a "double rainbow all the way".
Search youtube: Yosemitebear Mountain Giant Double Rainbow 1-8-10
Dude1: "Dude, how high were you?"
Dude2: "Dude, double rainbow all the way"
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