One of the mysteries of the English language finally explained.
Away from a microphone; when not speaking or performing directly into a microphone.
Not employing or speaking directly into a microphone; that is distant from, or not facing, a microphone; (hence, in extended use) that takes place behind the scenes; that is not part of a performance.
1930s. From off + mike. Compare slightly later off-microphone, off-microphone.
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