杯弓蛇影是一则由寓言故事演化而来的复句式成语。成语有关典故最早见于东汉学者应劭的《风俗通义·怪神》，《晋书·乐广传》又有类似故事记载。杯弓蛇影原义是将映在酒杯里的弓影误认为蛇，后比喻因疑神疑鬼而引起恐惧。

《古代寓言·杯弓蛇影》

予之祖你郴，为汲令，以夏至日诣见主簿杜宣，赐酒。

时北壁上有悬赤弩，照于杯，形如蛇。宣畏恶之，然不敢不饮。其日，便得胸腹痛切，妨损饮食，大用羸，攻，万端不为愈。复郴因事过至宣家窥视，问其变故。云：“畏此蛇。蛇入腹中。”郴还听事，思惟良久。顾见悬弩，“必是也！”则使门下史将铃下待徐扶辇载宣，于故处设酒，杯中故复有蛇。因谓宣：“此壁上弩影可，非有他怪。”宣遂解。甚夷怿。由是瘳平。

——《风俗通》

The Reflection of the Bow

My grandfather, who was magistrate of the district of Chen, once invited his secretary, Du Xuan, to drink with him during the midsummer festival. A red bow which was hanging on the north wall cast a reflection in the cup just like a snake; but although Du Xuan was frightened he dared not refuse to drink. Then he had a severe pain in his stomach, and could not eat, so that he grew very thin. Though he tried all manner of drugs, he could find no cure.

Later my grandfather called on him on some business, and asked him how he had contracted this illness.

"Through fear of the snake which I swallowed," Du Xuan told him.

After going home my grandfather thought this over, then turned and saw the bow, and understood what had happened. He sent a subordinate with a carriage to escort Du Xuan to his house, and set wine in the same place, so that once more a snake appeared in the cup.

"This is simply a reflection of that bow on the wall," he told his secretary. At once Du Xuan felt better and, greatly relieved, recovered.

--Traditional Topics (Feng Su tong)