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1.
to be high on marijuana aka pot, reefer, gange, etc
Dude, i got fucking diagonal last night
by Skottish June 08, 2007
 
2.
When double dating, the person you are not directly involved with.
I dunno whether to say she's my girl's friend, or my friend's girl. She's my diagonal.
by WADK November 15, 2007