beside oneself Definition

beside
prep

1. Next to, by the side of or near something or someone.

Thesaurus: by, next to, adjacent to, contiguous with, parallel to, adjoining, alongside, abreast of, neighbouring, overlooking, near, juxtaposed with.
2. Not relevant to something.

Example: That is beside the point
3. As compared with something or someone.

Example: All beauty pales beside hers

Idiom: beside oneself

In a state of uncontrollable anger, excitement or other emotion.
Example: By the time he arrived, I was beside myself
Example: beside herself with worry

Etymology: 13c: from Anglo-Saxon be by + sidan side.

Info:

There is often confusion between beside and besides: beside means ‘next to, at the side of', whereas besides means ‘in addition to, other than'.