There is definitely more than one answer to this question, but I would consider the following:
Chief's physical description of the nurse, particularly of her chest. She has very large breasts, which are obviously symbolic of femininity and motherhood. However, she goes out of her way to hide them, along with any other type of "softness."
Chief's hallucinations. Chief sees the nurse at one point as a huge monster who can reach out and grasp people with her arms. He believes that she has a bag full of mechanical parts that she uses to make alterations/installations in the patient's minds.
Her relationships with men. It seems that the nurse is in control of men in the text (even men like Dr. Spivey, who rank above her). What could this signify given that the story is set in the 1950's?
Her treatment of non-conformists. If a patient doesn't fly right initially, the nurse finds a way to make him conform. She ultimately destroys Macmurphy, the only one who is truly able to resist her.
If you consider all these ideas together, you'll probably figure out what the nurse represents.