This quote is pulled from the middle of Chapter 18 of To Kill a Mockingbird. (In the paperback edition published by Warner Books, it is on page 185).
In this chapter, Mayella Ewell is on the witness stand, supposedly recounting what happened on the day that she was purportedly raped. She is a fairly hostile witness as she complains after Atticus calls her "ma'am":
"I don't hafta take his sass. I ain't called upon to take it."
Judge Taylor explains that Mr. Finch is merely being polite. So Atticus continues and asks Mayella a number of questions. From this interrogation it becomes apparent that Bob Ewell is an irresponsible, dissolute, and neglectful parent. Their relief check is not only insufficient to provide for everyone, but Ewell often takes it and spends it on liquor. He stays away from the house and drinks until there is nothing left. The children fend for themselves, and as the oldest, Mayella must care for the younger ones.
When Atticus asks her, "Does he ever go after you?" Mayella becomes quiet. Judge Taylor orders her to answer. Then, she declares, "My paw's never touched a hair o' my head in my life." Clearly, she fears retaliation.