Completely unprepared or unplanned; done on the spur of the moment without any time for preparation.
From Latin ex- (completely, utterly) + temporalis (of time) + -aneous (suffix meaning 'full of'). It means literally 'entirely lacking in time,' implying no preparation or forethought.
This is basically 'extemporaneous' with extra emphasis—it's the difference between giving a speech off-the-cuff versus giving one with literally zero seconds of notice whatsoever.
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