From Longman Dictionary of Contemporary Englishdeadpandead‧pan /ˈdedpæn/ adjective EXPRESSION ON somebody'S FACEsounding and looking completely serious when you are saying or doing something funnydeadpan voice/expression etc deadpan humour
Examples from the Corpus
deadpan• This, and the fact that Mike was a competitor, seemed to get Dmitri edgy: his face went entirely deadpan.• "We're out of gas, so I guess you'll have to walk home, " he said, giving me a deadpan expression.• a deadpan expression• Garcia turns in another soulful-eyed but deadpan performance.• The third lady who wants more than lunch is Bowles, who describes their flight from convention in deadpan style.Origin deadpan (1900-2000) dead + pan “face” ((1900-2000)) 