Amid all the controversies and highly philosophical arguments surrounding Longstreet's entire life, the simple yet puzzling mystery of both his name (whether or not he had a middle name) and his moniker "Old Pete" remain. Not even Jeffry Wert, who perhaps wrote the most complete biography of Longstreet to date, addressed the issue in any detail. This may be because Wert limited his story to incidents and circumstances which could be proven through close study of primary evidence beyond doubt. Indeed, little primary evidence regarding Longstreet's name exists. In truth this fact is hardly surprising since the origin of names and nicknames is not something generally considered important to the world until that person becomes famous later in life.
The first question -- of Longstreet's middle name -- can be answered without a doubt. He didn't have one. This was an unusual thing at the time, and Longstreet often received letters which inquired as to his middle name, and in other dispatches to him, an arbitrary middle initial was often included. This can be attributed to the embarrassment some must have felt in not knowing Longstreet's middle name and not want to be viewed as impolite. They could have well written off an incorrect initial as a mere error in transcription. The irony, of course, was that in including an initial for such a reason indicated their lack of knowledge of the person to whom they were writing.
The second question, Longstreet's nickname, is more difficult. Although the evidence as to the origin of his nickname is somewhat anecdotal, one has to consider the source of the anecdote in relation to its potential truth. Some claim that "Pete" was applied to Longstreet while at West Point, but little evidence is offered to support this position. As reported by Glen Tucker, no less an authority than Mrs. Fitz Randolph Longstreet, Longstreet's daughter-in-law, offers probably the most likely explanation when she says that Longstreet's father was impressed early by his bearing and strength and gave him the name Peter because it meant "rock." This possibility is given further veracity when one considers that children are often given nicknames that have no relation to their birth names by their parents at an early age. Sometimes the name will stick throughout life, sometimes not.
Perhaps the reason Longstreet continued to be called Pete was that he had no middle name and thus "Peter" was often substituted by those around him. As he grew older and more mature, the addition of "Old" became a natural occurrence.
Source: Brian Hampton