The term 'Great Disappointment' refers to a period of time in the 19th century when a religious movement known as Millerism, which was led by Baptist preacher William Miller, predicted that Jesus Christ would return to Earth on October 22, 1844. When this did not happen, it caused a great deal of disappointment among Miller's followers, who had been expecting the Second Coming of Christ. The crossword solution for this term is 'BLOW, ' which can be interpreted as a metaphor for the crushing disappointment felt by Miller's followers.