为什么英国国旗的英文翻译是"Union Jack"？

英国国旗也称联合王国国旗。英语 是 the Union Jack．或Union Flag。Union指1606年英格兰和苏格兰的联合。Jack意为悬挂在船首表示国籍的小旗。

联合王国国旗(英国国旗)
union jack
Jack是海军用语,指悬挂在舰首的旗帜,英国军舰舰首都悬挂国旗,因而得名。
The issue of whether it is acceptable to use the term "Union Jack" is one that causes considerable controversy. Although it is often asserted that "Union Jack" should only be used for the flag when it is flown as a jack (a small flag flown at the bow of a ship), it is not universally accepted that the "Jack" of "Union Jack" is a reference to such a jack flag; other explanations have been put forward [1]. The term possibly dates from the early 1700s, but its origin is uncertain. The word Jack may have come from the name of the James VI, King of Scots who inherited the English crown, causing the flag to be designed, that is Jac from Jacobus, Latin for James. The size and power of the Royal Navy internationally at the time could also explain why the flag was nicknamed the "Union Jack" considering the navy was so widely utilized and renowned by the United Kingdom and Commonwealth countries it is possible that the term "Jack" did occur due to its regular usage on all British ships using the "Jack Staff" (a flag pole attached to a ship on the bow). Even if the term "Union Jack" does derive from the jack flag (as perhaps seems most likely), after three centuries, it is now sanctioned by usage, has appeared in official usage, and remains the popular term. The BBC website disregards the term "union flag" because of its "great potential for confusion", preferring union jack (in lower case)[2] The term "Union Flag", on the other hand, is the term preferred in official documents by vexillologists. The Merchant Shipping Act 1995[3] refers to the national colours of the United Kingdom as "the Union flag (commonly known as the Union Jack)".