Which president was known as "the Gentleman Boss" and why was he called that?

President Chester Arthur was known as "the Gentleman Boss".

Chester Alan Arthur (October 5, 1829 – November 18, 1886) was an American attorney and politician who served as the 21st President of the United States (1881–85); he succeeded James A. Garfield upon the latter's assassination.

Arthur is remembered as one of the most society-conscious presidents, earning the nickname "the Gentleman Boss" for his style of dress and courtly manner.

https://en.wikipedia.org/wiki/Chester_Arthur