Early, Modern Early Modern form of beside oneself.
Feſtus ſayde with a lowde voyce: Paul / thou arte beſides thy ſilfe. Moche learnynge hath made the mad.1526, [William Tyndale, transl.], The Newe Testamẽt […] (Tyndale Bible), [Worms, Germany: Peter Schöffer], →OCLC, The Actes off the Apostles xxvj:[24], folio cxcv, recto:
Notwithſtondinge it came ſo often vnto him / that what with laboure and whate with feare / the man was almoſt beſides him ſilfe / and then was he ſent to Oxfoꝛth to aſke counſell what was beſt to be done.c. 1531, John Frith, A diſputaciõ of purgatoꝛye [A Disputation of Purgatory]:
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For I am under the same accusation with my Saviour—for they said, he is besides himself.1759–c. 1763, Christopher Smart, Jubilate Agno; first published as W. F. Stead, editor, Rejoice in the Lamb: A Song from Bedlam, 1939: