The phrase 'Beside Oneself' describes a distressed person in an extreme emotional state, brought by a situation that causes one to be out of his wit and senses.
Example of use: "I know Daniel saw your girlfriend at the movies last night with another man, but there's no reason to be beside yourself, he's probably her cousin."
The first recorded use of the term is in the Old testament (Acts 26:24): "Paul, thou art beside thyself; much learning makes thee mad." The meaning of 'Beside Oneself' in the verse is that Paul's anger has driven him to the point of madness, however nowadays it describes a situation where a person is at an extreme mode of an emotion, such as happiness, sadness, madness or any other emotion.