Answer: Prejudice
Step-by-step explanation:
In 1954 Gordon Allport published a book titled 'The Nature of Prejudice' where he defined the term prejudice as "an antipathy based on faulty and inflexible generalization. It may be felt or expressed. It may be directed toward a group or an individual of that group".
The cashier judges his elderly customer based on the assumption that due to his age the customer will take time to pay for his goods. The cashier is using a stereotype however at the time he is using it to the extreme through his unhappiness of seeing the elderly customer at his till. The extreme stereotype is prejudice which is described by Allport.