杞人忧天
[词典] like the man of Qi who was haunted by the fear that the sky might fall -- to entertain imaginary or groundless fears; groundless worry; In the state of Qi there lived a man who, filled with apprehensions of the imminent fall of the sky and the consequent destruction of his hearth and home, was so vexed that he could neither eat nor sleep.; like the man in the state of Qi worrying in case the sky should fall; Meet troubles half-way;
[例句]可怜的唐纳德一直在杞人忧天。
Poor Donald's been worrying himself silly.