Final answer:
Moira cited a headache as an excuse to avoid a dinner engagement, likely due to social difficulties she was experiencing.
Step-by-step explanation:
Moira used the excuse of having a headache to get out of the dinner. The narrative suggests her difficulty in social situations, which may have contributed to her invention of the excuse. Bernice, the main character, encounters her cousin Marjorie's judgment and faces a disappointing social experience, which leads to her claiming to have a headache to avoid further social obligations. Such avoidance also leads to her eventual decision to leave, despite the emotional cost and the implications it may have on her relationship with her family.