beside oneself - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
In a state of extreme agitation or excitement, as in She was beside herself when she found she'd lost her ring, or Peter was beside himself with joy—he'd won the poetry award. This phrase appears in the New Testament (Acts 26:24): “Paul, thou art beside thyself; much learning makes thee mad.” [Late 1400s]