straight‧faced /ˌstreɪtˈfeɪst◂/
adjective without smiling or laughing, even though you are joking or saying something untrue:
‘No, I really do love you,’ said Kim, straightfaced.—straightfacedly /-ˈfeɪsədli◂, -ˈfeɪsɪdli◂/
adverb [TahlilGaran] Dictionary of Contemporary English ▲