keep a straight face
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]
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The American Heritage® Idioms Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.