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1.
1. A prefix used to create the superlative form of an adjective or adverb. Also used for emphasis to stress an idea/emotion. Is superior to the prefix "plus."
ex.
doubleplusgood = fantastic/wonderful/amazing/etc.
plusgood = great/fine/better/etc.

This term originated from George Orwell's novel, 1984 as a part of the modified English language of Newspeak. In the book, doubleplus was used to eliminate "unnecessary" words to limit one's vocabulary/ability to express.
from the book...
"If you want a stronger version of "good", what sense is there in having a whole string of vague useless words like "excellent" and "splendid" and all the rest of them? "Plusgood" covers the meaning, or "doubleplusgood" if you want something stronger still. "

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Larry: Did you just see that three pointer I just made? It was doubleplussweet.

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Gregizor: Our IB internal assessment is due tomorrow!? This is doublepluslame!!
by The Legendary Ironwood June 11, 2006
 
2.
Very or more in newspeak. More than plus, used to accentuate words.
That bus was doubleplus speedful.
by Jeremy Meerkreebs December 15, 2003