flexer

From flex +‎ -er.

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flexer (plural flexers)

1. One who flexes something physically.
   • 1963, Popular Mechanics, volume 119, number 4, page 80:

     Muscle flexers and weight lifters have been given academic approval by a University of California scientist.

2. One who flaunts or shows off.
3. (social media) diminutive of influencer