Compel someone to act or speak prematurely or against his or her will. For example, He didn't want to decide just then, but the board forced his hand. This expression probably alludes to the hand (the cards) held in a game such as whist or poker, in which a player is compelled to play some card from his hand or to reveal his hand. [Mid-1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.