A digram is when you have two letters in one box. These show up roughly once every 5 or so rounds. Create a word using the digram for the achievement.
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WARNING: Use the Wordament Solver at your own risk. The developers haunt the leaderboards for anything suspicious. They will ban you if they suspect you have cheated and you will then no longer be able to get the achievements.
Wordament Solver - http://dev.360surface.net/Wordament/EnterBoard.aspx