What did the 'S' in Harry S. Truman stand for?

It stood for 'S'
His mother and father each had a different middle name for him.
Both of them started with 'S' and since they couldn't decide, it was just left as 'S'

Truman did not have a middle name, but only a middle initial. It was a common practice in southern states, including Missouri, to use initials rather than names. Truman said the initial was a compromise between the names of his grandfathers, Anderson Shipp Truman and Solomon Young. He once joked that the S was a name, not an initial, and it should not have a period, but all official documents, and his presidential library all use the name with a period.
http://www.wikipedia.org/wiki/Harry_S_Truman

Truman did not have a middle name, but he did have a middle initial. Truman explained that the S was a compromise tribute to his grandfathers, Anderson Shipp Truman and Solomon Young. For years there has been a controversy over whether or not the S should have a period after it; the Harry S. Truman Library does use the period, as does the Government Printing Office, but many people don't because the initial doesn't stand for a name.
http://www.pbs.org/wgbh/amex/presidents/nf/teach/truman/trumandk.html

Below taken from site
http://www.usa-presidents.info/truman.htm


Truman's middle initial

Truman did not have a middle name, but only a middle initial. It was a common practice in southern states, including Missouri, to use initials rather than names. Truman said the initial was a compromise between the names of his grandfathers, Anderson Shippe Truman and Solomon Young. He once joked that the S was a name, not an initial, and it should not have a period, but all official documents, and his presidential library all use the name with a period. The Harry S. Truman Library states publicly that it has numerous examples of the signature written at various times throughout Truman's lifetime where his use of a period after the "S" is very obvious.

He had no middle name, the S was added later.