(a.) Composed, performed, or uttered on the spur of the moment\, or without previous study; unpremeditated; off-hand; extempore; extemporary; as, an extemporaneous address or production.

ad lib ad libitum ad-lib casual caught napping caught off balance caught short extemporary extempore extemporized haphazard hasty impromptu improvisatorial improvise improvised improviso impulsive informal jury-rigged makeshift off the cuff off-the-cuff offhand precipitate rough-and-ready snap spontaneous spur-of-the-moment stopgap surprised taken aback taken by surprise taken unawares tripped up unarranged unbegun unconcocted uncontrived undeliberated undevised unhatched unmade unmanufactured unorganized unplanned unpremeditated unprepared unprimed unready unrehearsed unstudied