"Real nobility is based on scorn, courage, and profound indifference" (Camus). Where can this definition be found in The Stranger?
There is no direct textual reference to "noble" or "nobility" in Camus' The Stranger. Camus rejected the idea that he was an existentialist but sought to find a unity between an absurd world and a person's actions in it. This quest is the backdrop for the comment that "nobility is based on scorn, courage, and profound indifference." This definition of "nobility" is what Camus sketches through Meursault's experience.
Based upon the falling action in The Stranger, it seems that one additional component to nobility is being able to express oneself and be attended to clearly. Meursault stands up in court and tries to explain that it was the heat that prompted him to kill the Arab, but he sits back down knowing that he has failed because his communication was unclear:
I tried to explain that it was because of the sun, but I spoke too quickly and ran my words into each other. I was only too conscious that it sounded nonsensical ....
In reference to the nobility quote, Meursault demonstrates his nobility in what might be called his extraordinary courage in standing up against the unrelenting heat from the Sun--until the fateful moment when he loses his courage, and with it his nobility, and yields to the Sun's power.
Meursault further demonstrates his nobility in reference to the nobility quote when he expresses his indifference and scorn--apathetic indifference and scorn--for the rituals and expectations of people and society. First, he scorns the conventions of the death vigil, then he scorns the conventions of the marriage union, to both of which he is wholly indifferent. Camus is protesting the absurd ironical inability in life to successfully demonstrate nobility in an absurd universe that backlashes against expressions of nobility.