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a reduction of outherwhere, autherwhere, from outher + where, the contraction being the same as in outher, our, ather, ar, either, er, other, or, whether, wher. The etymological sense was thus ‘either-where’, i.e. ‘either one where or the other’, ‘somewhere or other’, and thus at length = owhere, anywhere.
It is possible that our- or ouer- was later associated with over, and so with such combinations as overall, overall-where, whence perhaps 'everywhere'; but the northern forms in awre-, aure-, could be derived only from awther.
From: A New English Dictionary on Historical Principles, James Murray, 1888-1933