It's a shortening of Captain General - which is one of those weird  titles where the adjective comes after the noun, like attorney general or procurator fiscal  - rather than just being General Captain.

它是 Captain General 的缩写——这是一个奇怪的头衔， 形容词位于名词之后，例如 Attorney General 或 Procurator Financial——而不仅仅是 General Captain。