It's similar to how you relate to a kid where you recognize yourself as an adult and them as a child, so you don't personalize or internalize their actions. Doctors and patients, even developers and users. You can be equitable about it by accepting things as they are, but internally, you can do that because you don't need for them to reflect your own self belief.
Among adults, it's recognizing people as true others, who are not a part of "your," tribe or experience, just neutral independent beings. In a professional context, their opinions are objective things that you don't register personally because your sense of self does not depend on your relationship to them. People will bend over backwards trying to explain it in a non-hierarchical neutral zen way, but the example makes it easier.
It comes from working with animals, where a trainer needs to emotionally separate themselves from the animals actions so you cease to relate to them an a reactive way.
On mobile so can't source it for you now, but it originated either in a translation of General L'Hotte or Baucher. However you would be surprised how many words and usages cannot be found on Google or in modern dictionaries.
What people call self confidence comes from experience or beliefs, and few people who encounter it can tell the difference.
