unkindest cut

The worst insult, ultimate treachery, as in And then, the unkindest cut of all—my partner walks out on me just when the deal is about to go through. This expression was invented by Shakespeare in describing Julius Caesar's stabbing to death by his friends in Julius Caesar (3:2): “This was the most unkindest cut of all.”

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.