flex one's muscles
Show off one's strength or power, as in The boys love flexing their muscles, or The new department head has decided to flex her muscles. [Early 1900s]
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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.