straight face
noun
: a face giving no evidence of emotion and especially of merriment
lied with a straight face
straight-faced
ˈstrāt-ˈfāst 
adjective straight-facedly
ˈstrāt-ˈfā-səd-lē
adverb -ˈfāst-lē
Example Sentences
She lied with a straight face. The students quickly put on straight faces as the teacher walked into the room.
Recent Examples on the Web Goins could hardly keep a straight face when the ever-inquisitive Michael asked her about the morning routine. 
Kyle Neddenriep, The Indianapolis Star, 31 Aug. 2022 Wave sits up with a straight face, dressed in a ribbed shirt, striped pants and a yellow bib with a bee on it. Angela Andaloro, Peoplemag, 16 Aug. 2022 One brave celebrity shows up each week without a script, trying to keep a straight face while ad libbing alongside Will Arnett’s hilariously vain homicide detective Terry Seattle. Los Angeles Times, 18 May 2022 Who can keep a straight face when guests are using food like musical instruments? Karla Pope, Woman's Day, 4 May 2022 Sankey’s not the one to answer that with a straight face. Joseph Goodman | Jgoodman@al.com, al, 18 July 2022 Gone are the days when Hastings could say with a straight face that Netflix didn’t compete with companies like Apple and Amazon. Walter Frick, Quartz, 8 June 2022 No longer can anyone say with a straight face that Anglo-American liberal democracy is a timeless good, or the inexorable goal of all modern nation-states. Krithika Varagur, The New Yorker, 25 May 2022 Jeong could be seen with his arms crossed and a straight face as Giuliani spoke. Ryan Gajewski, The Hollywood Reporter, 20 Apr. 2022 See More
Kyle Neddenriep, The Indianapolis Star, 31 Aug. 2022 Wave sits up with a straight face, dressed in a ribbed shirt, striped pants and a yellow bib with a bee on it. Angela Andaloro, Peoplemag, 16 Aug. 2022 One brave celebrity shows up each week without a script, trying to keep a straight face while ad libbing alongside Will Arnett’s hilariously vain homicide detective Terry Seattle. Los Angeles Times, 18 May 2022 Who can keep a straight face when guests are using food like musical instruments? Karla Pope, Woman's Day, 4 May 2022 Sankey’s not the one to answer that with a straight face. Joseph Goodman | Jgoodman@al.com, al, 18 July 2022 Gone are the days when Hastings could say with a straight face that Netflix didn’t compete with companies like Apple and Amazon. Walter Frick, Quartz, 8 June 2022 No longer can anyone say with a straight face that Anglo-American liberal democracy is a timeless good, or the inexorable goal of all modern nation-states. Krithika Varagur, The New Yorker, 25 May 2022 Jeong could be seen with his arms crossed and a straight face as Giuliani spoke. Ryan Gajewski, The Hollywood Reporter, 20 Apr. 2022 See More Word History
First Known Use
1853, in the meaning defined above