: to bend especially again and again <Can you flex your fingers?>
: to move or tense (a muscle)
Word Root of FLEX
The Latin word flectere, meaning “to bend” or “to curve,” and its form flexus give us the roots flect and flex. Words from the Latin flectere have something to do with bending or curving. To flex is to cause something, such as a muscle, to curve or bend. Something flexible can be bent without breaking. To deflect, or turn aside, is to bend the direction of something. To reflect is to bend an image back to the person looking at a mirror or other shiny surface.
Medical Definition of FLEX
: to bend especially repeatedly
a: to move muscles so as to cause flexion of (a joint) <stretching and flexing his knees>b: to move or tense (a muscle or muscles) by contraction <flexed their biceps>