To move part of the body using one's muscles. (verb)
To tighten the muscles for display of size or strength. (verb)
Examples of word flex
CLANCY: And it's a whole industry, because you've to have your cars -- in Brazil, you find most of the cars are what they call flex vehicles, so they can either run on ethanol, or they can run on regular unleaded gas.
If you wanted all this dynamic html content you should have went with adobe flash (flex is da bomb).
They run deeper than Rules, allow for a bit of flex (let's face it, there isn't a Rule for every situation and sometimes flex is needed to allow for times when you need to bend one place to keep something more important intact), but still provide some boundaries, a compass for making good choices.
Challenging him made his expression flex with what she had then sensed and now knew to be hunger.
Then two soldiers on the ground led a hooded figure, his hands tied in flex cuffs and a number scrawled on his back, to the helicopter.