As I walked down 5th Street today, I saw a green Cooper Mini with the British flag painted atop its roof. Obviously, the vehicle’s owner is not interested in making America great again. Nevertheless, I wondered, “Why is that flag called the Union Jack?”

The website royal.uk did not answer my question. The page states that

The term ‘Union Jack’ possibly dates from Queen Anne’s time (r. 1702-14), but its origin is uncertain.

It may come from the ‘jack-et’ of the English or Scottish soldiers, or from the name of James I who originated the first union in 1603.

Another alternative is that the name may be derived from a proclamation by Charles II that the Union Flag should be flown only by ships of the Royal Navy as a jack, a small flag at the bowsprit; the term ‘jack’ once meant small.

https://www.royal.uk/union-jack#:~:text=The%20term%20%27Union%20Jack%27%20possibly,the%20first%20union%20in%201603.

Thanks a lot, wankers.