From Longman Dictionary of Contemporary Englishstraight facestraight faceSERIOUS/NOT JOKINGif you have a straight face, you are not laughing or smiling even though you would like to I found it very difficult to keep a straight face. → straight
Examples from the Corpus
straight face• For the first hundred yards we keep straight faces.• How can you say that and keep a straight face?• It was very difficult to keep a straight face.• Kemp is straining to maintain a straight face.• We start with safe conversation and straight faces.• I try and keep a straight face, but I can't help grinning at myself.• Despite the problem of trying to maintain a straight face, there are distinct advantages to being on Cube's team.• The old trick of keeping a straight face was failing him these days. 