black swan（n.）
“寓言式的”意思是“极其罕见或不存在的”，出现在14世纪后期，来自于朱文尼亚尔（Juvenal）[ "Sat." vi. 164]，但真正的翻版后来在澳大利亚发现（Chenopsis atratus）。
"Do you say no worthy wife is to be found among all these crowds?" Well, let her be handsome, charming, rich and fertile; let her have ancient ancestors ranged about her halls; let her be more chaste than all the dishevelled Sabine maidens who stopped the war—a prodigy as rare upon the earth as a black swan! yet who could endure a wife that possessed all perfections? I would rather have a Venusian wench for my wife than you, O Cornelia, mother of the Gracchi, if, with all your virtues, you bring me a haughty brow, and reckon up Triumphs as part of your marriage portion. [Juvenal]
“你说在这些人群中找不到一位有价值的妻子？” 好吧，让她又漂亮、又迷人、又富有、又肥沃； 让她的古老祖先围绕着她的大厅； 让她比所有那些停战的沙宾女郎都更贞洁——这在地球上就像黑天鹅一样罕见！然而，谁能忍受一个拥有所有完美境界的妻子呢？如果你带着所有的美德给我带来了傲慢的眉头，并把胜利归功于你的婚嫁礼品，那么我宁愿把你换成维纳斯的女仆，而不是你，格拉克二世的母亲，哦，科涅利亚 [朱文尼亚尔]。
Blue dahlia 在19世纪也用于“罕见且闻所未闻的东西”。
该词起源时间：14世纪末