Цитата сообщения el_flaco Words and their Stories - BESIDE ONESELF

Many expressions may be used to describe someone who is excited.
One such expression is to be "beside yourself." You can be beside yourself with anger or beside yourself with happiness, although usually not both at the same time. If you are beside yourself, you are in an extremely emotional condition. You are filled with excitement.
The dictionary tells us that "beside" means next to, or at the side of. So the expression "beside oneself" describes something that really is not possible. You cannot be next to yourself. Some language experts, however, think the expression probably comes from an old belief that through magic, you could indeed be next to yourself. Spirits were believed to have the magic power to do anything. So it was possible to have two of the same person appear, especially if that person was excited.
Today, you might hear a husband say, "When my wife told me we were going to have a baby, I was beside myself with joy." Or someone might tell you he was beside himself with anger because he had just lost his job.
When you are full of joy or are extremely excited about something you may do something else that is strange. You may "flip your lid." A lid is the cover that prevents something from escaping from a container. A lid on a cooking pot, for example, keeps the heat from escaping. To flip something is to turn it over. So, when you "flip your lid," you become so excited that your self-control escapes.
You can "flip your lid" over something you like very much. A young man, for example, might flip his lid over a pretty, young woman. Or you can "flip your lid" if something makes you very angry. If someone hit your new car, it might make you flip your lid.
In recent years, the word "flip" itself has come to mean the same thing as flip your lid. It is common to hear a girl say she "flipped" over a new boyfriend.
An expression that means something quite similar is to "lose your head." The head is believed to be connected to reason and careful thought. Thus to "lose one's head" is to act without thinking, to be out of control.

(c) VoA