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

The idiom "speak one's mind" dates back to the 16th century and refers to the act of expressing one's thoughts and opinions openly and honestly, even if they are controversial or unpopular. It is often used to convey a sense of courage and conviction, suggesting that the speaker is not afraid to say what they believe, even if it means causing offense or upsetting others.