Meaning of triple witching hour

tri'ple witch'ing hour"

Pronunciation: [key]
  1. the last hour of trading on the New York Stock Exchange on the four Fridays each year when stock options, stock index futures, and options on such futures simultaneously expire: regarded as a time of extreme volatility in trading.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.