What is the source of human dignity?

This is a sort of philosophic question (the quality or state of being worthy, honored, or esteemed) and so is best answered as ... it's a state of mind. There is no reality behind the concept beyond our own beliefs.

Human dignity is both self evident and mysterious. Martin Luther King answered the question this way: "Each person possesses an inviolable sacredness by virtue of being called into existence and sustained by God." Aristotle identified the source of human dignity in the persons ability to reason; man is a rational animal. A third source of human dignity is the recognition that every person is irreplaceable and unrepeatable.