What is the origin of the idiom "flex one's muscles" and when to use it?

The idiom "flex one's muscles" originated from the literal action of flexing or contracting one's muscles to demonstrate strength or physical prowess. The term "flex" refers to the action of tightening or contracting muscles, typically done to showcase their size, definition, or power. Over time, the phrase became a figurative way to describe assertive displays of power, skill, or authority in different domains.