beside oneself

Possibly derived from Ancient Greek παράνοια (paránoia, “madness”), from παράνοος (paránoos, “demented”), from παρά (pará, “beyond, beside”) + νόος (nóos, “mind, spirit”), with "beside mind" evolving into "beside oneself".

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beside oneself

1. (idiomatic) Overcome; consumed by an emotion.

   His widow was beside herself with grief.

   • 1860, Henry David Thoreau, The Last Days of John Brown‎:

     [...] he contemplated it most calmly. Comparatively, all other men, North and South, were beside themselves.