DQ is short for disqualified. It gets its name from the way it appears the rolling scores reports, starting from Round 235 (August 1991); 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 (that is, informing the dealer) 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.