straightfaced
From Longman Dictionary of Contemporary Englishstraightfacedstraight‧faced /ˌstreɪtˈfeɪst◂/ adjective SERIOUS/NOT JOKINGwithout smiling or laughing, even though you are joking or saying something untrue ‘No, I really do love you, ’ said Kim, straightfaced. —straightfacedly /-ˈfeɪsɪdli◂/ adverb