The term DQ doesn’t appear anywhere in the rules. But it’s short for disqualified (though the rules themselves mostly use the word disabled); it gets its name from the way it appears in the rolling scores report; and it arises from this rule:
If at any time before you vote you come, by any means, to know the definition of The Word, you are disqualified from voting and from offering commentary. ... If you know The Word … you should … inform the dealer ... .
This paper maintains that DQing is often messy, that it can result in unintended spoiler messages, that it serves little purpose, that it has assumed a spurious importance in the gameplay, and that players would often do better just to sit out the round.