extemporaneous       
adjective  formal uk/ekˌstem.pəˈreɪ.ni.əs/ us/ekˌstem.pəˈreɪ.ni.əs/
 即兴的;无准备的;即席的
done or said without any preparation or thought
  an extemporaneous speech 即兴演讲 
 Synonyms: extempore impromptu
 
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