why is CX called Cathay Pacific?

Each airline has its own code. Normally, it is abbreviation of its nam, Northwest (NW), British Airways (BA) etc. Thus, CX is the abbreviation of C(C)athay Paci(X)fic Airways.

Although now restricted mostly to poetic usage, the name Cathay has historically been used as a name for China , particularly its northern half. Cathay derives from the name of the Khitan (契丹) ethnic group, which ruled over most of North China as the Liao Dynasty until being displaced in turn by the Jurchens from the northeast and the Mongols from the north. A cognate form, Kitay, remains the most commonly used name for China in the modern Slavic language.

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Very good answer, Cathay 的意思是 China.