Woot! A GQ I actually know the answer to!
Like Harry Truman, the “S” stands for nothing.
More than you ever wanted to know about the subject, excerpted from “Echoes”, Published by The Ohio Historical Society in April, 1963, can be found here:.
http://www.lib.siu.edu/projects/usgrant/newsletter/newsletter.html
In short, his name was originally drawn out of a hat(!) and was Hiram Ulysses Grant. His family called him “Ulysses”, and his mates called him or “Lyss” or, showing the sensitivity young ones are famous for, “Useless”.
In 1839 Grant’s dad asked Congressman Thomas L. Hamer to appoint Ulysses to West Point. For reasons unknown, Hamer, rather than simply nominating “Ulysses Grant”, which would have pleased everyone, inserted a spurious middle initial “S” (no “Simpson”, just “S”). Why? No one knows.
Further muddying the waters, Grant himself decided to change his own name to avoid confusion (“My name is Hiram but call me Ulysses”). So Hiram Ulysses Grant, who was appointed to West Point as Ulysses S. Grant, signed in as Ulysses Hiram Grant.
I chuckle trying to imagine the conversation with authority when Grant tried to sort this all out, and I sympathize with whatever officer got a headache listening to this tale and apparently told him that “Ulysses S. Grant” was appointed to West Point, and Grant could either be that person or go home. At any rate…
Quote:
…in the fall he signed a certificate of enlistment as U. S. Grant. Although the army had given him a name he had to accept officially, for the four years of his cadetship he continued to sign his private correspondence U. H. Grant. After he had received a diploma and a commission as Ulysses S. Grant, however, he abandoned his chosen name for the army issue. His classmates had used the initials anyway, and called him “Uncle Sam” at first, but later settled on “Sam”.
<<SNIP>>
In a letter to his special friend and patron, Congressman Elihu B. Washburne of Illinois, Grant gave an indication of his annoyance: “In answer to your letter of a few days ago asking what ‘S’ stands for in my name, I can only state nothing .”
In spite of the fact that Grant never referred to himself or signed his name as Ulysses Simpson Grant, it is still persistently believed that this was his name. Unlike Harry S. Truman, who has no middle name and has defended his right to a simple initial, Grant was too reticent to correct widespread public error. The man who consistently signed himself U. S. Grant patiently bore a name which he did not acknowledge as his own.
End Quote.