What does Aunt Alexandra mean by "Somebody just walked over my grave"?

In order to understand the significance of the phrase in the book, we must first look into the background of the phrase itself. There is an old wives' tale relating that one would receive a sudden chill when walking over the place where his or her grave would one day be. The first documented use of the phrase "There's somebody walking over my grave" was in in the first half of the 1700s, used in response to a shiver or getting goose bumps, referring to the aforementioned superstition. In To Kill a Mockingbird, Aunt Alexandra is described as receiving "a pinprick of apprehension", suggesting that the phrase had further evolved to mean being overcome with a peculiar feeling. Beyond the origins of the phrase, Aunt Alexandra's quote is significant because it foreshadows the events that occur after the Halloween pageant, when Bob Ewell attempts to harm Scout and Jem. Alexandra gets a sudden feeling that something could go wrong, but she shakes it off and allows the children to attend the pageant unattended.