What is the origin of the idiom "deep down" and when to use it?

The idiom "deep down" originated from the concept of depth as a metaphorical representation of one's true or innermost self. It signifies the underlying or fundamental nature of a person, belief, or emotion that exists beyond surface appearances or initial impressions. The phrase is used to emphasize the idea that there are hidden or profound aspects within individuals that may not be immediately apparent but play a significant role in shaping their thoughts, feelings, or motivations.