to bend, as a part of the body: He flexed his arms to show off his muscles. to tighten (a muscle) by contraction. to demonstrate (one’s ability, expertise, etc.): She’s flexed her marketing skills for various clients in the publishing industry. Slang. to boast or brag about; flaunt: He’s just not into flexing his success.

to bend an arm, leg, etc. or tighten a muscle: First, straighten your legs, then flex your feet. He tried to impress me by flexing his huge muscles. [ I or T ] (of a material) to bend without breaking, or to make a material do this: Sheets of brittle lava broke under their own weight as they flexed. Metal fatigue occurs when steel is flexed.

There are three main reasons you can use the slang ‘flex’ — while referring to a genuine show-off, a fake show-off, and an awkward show-off. All can apply to yourself as well. A proud moment for someone can be perceived as a moment of flex by the audience.