This phrase can imply aggression or a challenge, especially in political or social power contexts. The small nation decided to flex its muscles in the face of its larger neighbor's demands.
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Asserting Dominance
Used to describe actions taken to display control or superiority in a situation. The new manager flexed his muscles by implementing major policy changes.
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Intimidation
Can suggest an attempt to intimidate or exert influence over others. During negotiations, the corporation flexed its muscles to get a better deal.