He was beside himself with excitement

This conjures up amusing images of something physically impossible. It must really confuse foreigners trying to learn English.

I decided to look for the origins to see if they ever made any sense and this is what I found:

In ancient times people believed that when people were under emotional stress, their soul would leave their body. The phrase appeared in the New Testament (Acts 26:24):

Paul, thou art beside thyself; much learning makes thee mad.

Sources:

Dictionary of Word Origins” by Jordan Almond (Carol Publishing Group, Secaucus, N.J., 1998)

The American Heritage® Dictionary of Idioms by Christine Ammer.