By the pricking of my thumbs

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"By the pricking of my thumbs" is the first half of a line in Act 4, Scene 1 of William Shakespeare's play Macbeth. Uttered by the Second Witch, the line reads:

By the pricking of my thumbs,
Something wicked this way comes.
Open, locks,
Whoever knocks!

The notion of a pricked thumb came from ancient Rome. It was said by seers that palpitations of the heart, the flickering of the eye and the pricking of a thumb were all warnings of evil. In particular, a pricking sensation in the left thumb was very worrisome.