Also the boyhood nickname of Ulysses Grant, Jr., the second son of Ulysses Grant, because he was born in Ohio, the Buckeye State. The name was given to him before the war by slaves on Grant's farm in Missouri.
From The Language of the Civil War by John D. Wright page 43.
Oh what a hoot! A socialite nicknamed "Buck?" That's hilarious. Surely they knew what the term meant... "buck... strong, lusty." From a book on Americanisms, 1871. Of course it also had a connotation meaning "male negro." Wonder what kind of lady Sallie was?
A good book on Sarah Buchanan Preston is "A Silence After Trumpets: The Story of Sarah Buchanan Preston". It is by Frances Herman Jackson and Mary McNease Kinard. Sarah was a member of the prestigious Hampton-Preston family. She was a first cousin of Wade Hampton. She was also a friend of Mary Boykin Chestnut. Sarah, known as Buck to her friends, along with her two sisters, Mamie and Tudie were at the center of events in both her native Columbia and Richmond. Sarah was said to have classic beauty and had many suitors including John Bell Hood. She later married another and had several children. She died at an early age. There is a very nice memorial to her on find a grave.