The “S” in Ulysses S. Grant doesn't stand for anything!

His proper name was actually Hiram Ulysses Grant. It’s often said that he changed his name to Ulysses Simpson Grant, but that’s not actually true. Ulysses was just his middle name, and the S stood for nothing. 

People often think his middle name was Simpson because of his mother, Hannah Simpson Grant. Some also believe it stood for Sam, because of that being his nickname for a time in the military academy. 

So why did he add it? The truth is he just wanted his name to feel more distinguished. Grant was the 18th US President from 1869 to 1877, and was one of the more notable leaders in the American Civil War. 

His reputation as president is often thought pretty damaged by certain actions and failures, including the economic depression known as the Panic of 1873 and his failure to make the Reconstruction effort succeed as conservative white southerners regained control of every state in the South. Despite that, he still had success in nearly destroying the Ku Klux Klan in 1871. 

(Source)