"deadpan" in the adjective sense
1. deadpan, expressionless, impassive, poker-faced, unexpressive
deliberately impassive in manner
"deadpan humor"
"his face remained expressionless as the verdict was read"
"deadpan" in the adverb sense
1. deadpan
without betraying any feeling
"she told the joke deadpan"
Source: WordNet® (An amazing lexical database of English)
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