MERCURY IN THE TWELFTH HOUSE will tend to keep you back in life and not allow you to exercise your mind as fully as you should. You may suffer from secret enmity or slander, deserved or not. You will be in some temporary danger of mental affection at some time of life and it would be well not to let yourself be drawn into any unwise scheme, especially where writings are concerned. Be careful not to put your hand to papers unthinkingly, and never let your mind be overtaxed or worried as it would affect the brain. You might easily over-ride enmity by your speech and action.
Alan Leo
You don’t speak up readily, even when you have a lot to say! Also, your mind tends to drift and you find it difficult to study very factual material that doesn’t have much color or imaginative appeal. Your intuition and first impressions are likely to be quite accurate, however, and you tend to depend upon this faculty in making decisions.
Kepler Natal Report