Prick my thumb

"to prick my thumb"

Does any one know the meaning of this phrase? I've read it in Shakespeare (Macbeth) and it wasn't annotated. I have also heard Robert Graves say it in an interview.

"By the pricking of my thumb, something wicked this way comes"
Shakespeare's witch does not prick her thumb (poke it with a sharp point). Her thumb is pricking by which she means she feels a tingling sensation in her thumb (like pins and needles). This tingling of the thumb was taken as an omen of something about to happen.