Final answer:
The term supercilious describes an arrogant or disdainful attitude, often indicating a sense of superiority over others.
Step-by-step explanation:
The word supercilious means having or showing arrogant superiority to and disdain of those one views as unworthy. It is an adjective that describes someone who is overbearingly proud and contemptuous.
In the provided sentence, the term supercilious behavior refers to an attitude that is haughtily dismissive or looking down upon others, like the sister ignoring the janitor's greeting.
This term often signifies a sense of superiority and a lack of respect for others perceived as lower in status or quality.