deadpan       
adjective uk/ˈded.pæn/ us/ˈded.pæn/
 故作严肃的;假装正经的;不动声色的
looking or seeming serious when you are telling a joke
  a deadpan expression/voice 故作严肃的表情/嗓音 
 Synonyms: expressionlessimpassivepoker-faced
 
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