Flex /fleks/ verb  

Definition: Flex is a verb we use in the English language that has a multi-faceted meaning.

1. On one hand, the word flex can refer to physical body movements that cause a joint to bend or become bent.
2. Flex can also refer to the physical movement that causes muscles to become more prominent, i.e., flexing one’s muscles. 
3. When used as slang, flex is an informal way of saying that someone is showing off, flaunting valuable skills, or making a spectacle of themselves.

Etymology: The first use of the word flex is in 16th century Latin. Etymologists say the word was likely derived from Old French (before the 16th century) as flexible. They believe flex relates to the Latin verb flectere, which means bent. 

Regardless of their origin, all forms of the word flex relate to flexibility of movement or spirit and bending. Today, we use the term in English to describe someone who is showing off or giving an outstanding performance.

In a Sentence

The bodybuilder flexed his muscles for the judges during the bodybuilding competition.

Being able to earn multiple streams of income is a flex for today’s entrepreneurs.

During the exam, the doctor had the patient flex his elbow joint to test for mobility.

Synonyms 

Bend, Curve

Antonyms 

Tense, Straighten