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.
"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. "
Larry: Did you just see that three pointer I just made? It was doubleplussweet.
Gregizor: Our IB internal assessment is due tomorrow!? This is doublepluslame!!