The decision or course of action has been determined and cannot be changed. For example, Now that I've announced my resignation, the die is cast. This expression comes from the Latin Iacta alea est, “the dice have been thrown,” which according to Suetonius was said by Julius Caesar when he crossed the Rubicon and invaded Italy in 49 b.c. In English it dates from the first half of the 1600s.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.