Q: What is the meaning of the quote By the pricking of your thumb something wicked this way comes?
A: It means that something evil or terrible or "wicked" is on it's way. The witches in Macbeth (from whom this quote is said) can apparently detect evil by the cracklings in their thumbs.
This is similar to saying that when bad things are about to happen, the hair on your neck stands up straight.
When Catweazle's thumb is pricking he immediately knows somebody is approaching him. A book on superstition actually states that people used to believe an itchy or pricking thumb heralded visitors. Other superstition regarding the thumb is also described: people used to believe that a pricked thumb was a sure sign that evil was on its way. It was also believed that people with a long thumb were stubborn, and a wide thumb was said to indicate prosperity.