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Adjective: extemporary  ik'stem-pu,re-ree [N. Amer], ik'stem-pu-(ru-)ree or ek'stem-pru-ree [Brit]
  1. With little or no preparation or forethought
    "an extemporary lecture";
    - ad-lib, extemporaneous, extempore, impromptu, offhand, offhanded, off-the-cuff, unrehearsed

See also: ill-prepared, unprepared