Synonyms and related words :
ad-lib, caught napping, caught off balance, caught short, extemporaneous, extemporized, haphazard, hasty, impromptu, improvised, makeshift, precipitate, rough-and-ready, snap, surprised, taken aback, taken by surprise, taken unawares, tripped up, unarranged, unbegun, unconcocted, uncontrived, undeliberated, undevised, unmade, unmanufactured, unorganized, unplanned, unpremeditated, unprepared, unprimed, unready, unstudied