Flex is a slang expression for showing off with your assets, be it physical, financial or mental; it is to boast with your superior position to others.
People, who “flex” too much are considered rigidly arrogant, egocentric and exaggerated.
The term had been in use in African American Vernacular English as a slang term for showing valiance, or bragging since the 1990’s.
The expression appeared in songs like Ice Cube’s Today Was a Good Day since 1992.
The term comes from the act of flexing one’s muscles, to show the physical capabilities, often right before a clash.
This had been adopted in a more metaphorical way, to express the same meaning when people “flex” with their riches or intellect.
- Urban Dictionary – Flex