If a plan, remark, or move is ill-advised, it is not wise, and it is likely to cause trouble later. We use it more about the action than about the person: an ill-advised comment is common; calling someone ill-advised is possible but less usual. It is fairly formal. In everyday talk people more often say that was a bad idea. The feeling is not only “wrong in this moment,” but “this will probably come back to hurt you.” Though actually, advised here does not always mean someone sat you down and gave you advice. It means the thing was not carefully thought through — whether the problem was poor advice, no advice, or a rush of feeling. It is not related to being ill in the sense of sick.