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flex (one's) muscle(s)
1. Literally, to contract one's muscles, especially to demonstrate their size and subsequent strength. He's so vain, flexing his muscles in the mirror whenever he thinks no one is looking. She flexed her muscles and told us not to mess with her brother again.
2. By extension, to demonstrate one's influence, power, or strength. The small but outspoken nation has been flexing its muscles lately, with a wide range of military and ballistics demonstrations. The wealthy business tycoon flexed his muscle to get the charges dropped against his son.
See also: flex
flex (one's) musclesInformal
To exhibit or show off one's strength.