keep a straight face, to

To have an emotionless facial expression, especially in a humorous situation that could easily cause one to laugh. It's so hard to keep a straight face on camera when you have hilarious co-stars like I do.

Don't show one's feelings, especially refrain from laughing. For example, The school orchestra played so many wrong notes that I had trouble keeping a straight face . [Late 1800s]

COMMON If you keep a straight face, you manage not to laugh or smile. `I don't see that there's anything funny about it,' he said, offended. `Of course there isn't,' she said, trying to keep a straight face. Note: You can also say that you say or do something with a straight face. We've all been practising trying to say we charge $1,000 an hour with a straight face. Note: You can also say that someone is straight-faced. It's the way he tells a joke. He's completely straight-faced and I just fall about laughing.

Do not burst out laughing; look appropriately serious. This cliché is of fairly recent vintage. Henry Miller used the term in Plexus (1953): “All I felt called upon to do was keep a straight face and pretend that everything was kosher.”