From Longman Dictionary of Contemporary Englishkeep a straight facekeep a straight faceto not laugh or smile, even though something is funny → face
Examples from the Corpus
keep a straight face• With a completely straight face, Thomas announced he was joining the Girl Guides.• How can you say that and keep a straight face?• I try and keep a straight face, but I can't help grinning at myself.• I keep a straight face and the composure of a chemist dispensing a prescription.• The old trick of keeping a straight face was failing him these days.• It was very difficult to keep a straight face.• When reading some of the competition entries, it was hard to keep a straight face.• She looked so ridiculous it was hard to keep a straight face.• He looked so ridiculous -- I don't know how I managed to keep a straight face.• Barbara tried to keep a straight face, but in the end she just couldn't help laughing.• "I think I've lost that camera you lent me, " I said, trying to keep a straight face.• For the first hundred yards we keep straight faces. 