so long是“再見”的非正式表達，經常用在你跟對方要有比較長的時間不能見面的情況下，比如有人要去出差一段時間，你跟他再見的時候可以用so long。有點類似中文的“後會有期”。
So long, 在我們的理解中總覺得類似：“long time no see”
更像是見面寒暄的一種，但實際上so long則被西方人用來當“再見”講，但究竟為什麼用so long卻有許多不同的說法？
So where did it come from?（它源自哪裡？）
Some believe the phrase to be adapted from another language. One suggestion is that so long is from the Irish good-bye slán, which is not an unreasonable theory.
有些人認為這個短語是從另一種語言改編而來的。其中一個說法是，so long是出自愛爾蘭語的再見slán。這個理論聽起來似乎有一點道理。
Other possible sources that have been proposed include German adieu so lange (roughly translated: "farewell until we meet again"), Hebrew shalom (a greeting and a good-bye), and Arabic salaam (which is a greeting, not a good-bye, and is thus disqualified).
也有人提出其他的來源，包括德語的adieu so lange（大致翻譯為：告別，直到我們再次見面），希伯來語shalom（打招呼和再見），阿拉伯語salaam（是打招呼的用語，不是再見，因此這個說法並不可靠）
Except for having similar sounds, these terms, like Irish slán, have no provable connection to the English expression.
除了發音相似外，這些短語，像是愛爾蘭語slán，與英語表達都沒有可以證明的聯繫。
Usage history of so long suggests it could be an Americanism. Early use is found in Walt Whitman's poem about death and leave-taking entitled "So long!", which was published in the 1860 edition of his Leaves of Grass.
我們從歷史使用上找，結果發現這是一個美國用語。最早出現在沃爾特·惠特曼1860年出版的關於死亡和告別的詩集《草葉集》的一個標題。
While my pleasure is yet at the full, I whisper, So long! / And take the young woman’s hand, and the young man’s hand, for the last time.
當我的快樂已盡時，我低聲說，再見!抓住年輕女子的手和年輕男子的手，直到最後一刻
So long! Remember my words—I may again return, / I love you—I depart from materials; / I am as one disembodied, triumphant, dead.
再見！記住我的話——我會再回來。我愛你——我遠離物質。我是作為一個支離破碎的、成功的、死亡的。
It appears that the expression was not yet generally understood by Whitman's American contemporaries.
這句話似乎還沒有被惠特曼同時代的美國人普遍理解。
A friend of his hadn't heard it until he read the poem, and when he asked Whitman to define it, he called it
他的一個朋友從來都不知道這個說法，直到讀了這首詩，當他要求惠特曼給它下定義時，他稱它為
"[a] salutation of departure, greatly used among sailors, sports, & prostitutes—the sense of it is till we meet again—conveying an inference somewhere, some how [sic] they will doubtless so meet—sooner or later."
離開時的致意，多用於水手、運動員和失足婦女——它的意思是直到我們再次相見——傳達了一種不知何地，不知如何，他們早晚都會再見面的意思。
Wherever the expression originated, it diffused quite rapidly. By the 20th century, so long became a common expression of farewell—and no one knows how it happened.
無論這種表達起源於何處，它都傳播得很快。到了20世紀，so long變成了告別的常用表達，沒有人知道它是如何發生的。
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