unkindest cut of all, (most)

unkindest cut of all, (most)

The worst insult, the ultimate in treachery. This expression was used by Shakespeare to describe the assassination of Julius Caesar by his friends: “This was the most unkindest cut of all” (Julius Caesar, 3.2). It found its way into the proverb collections of James Howell, John Ray, and Thomas Fuller, which helped its long survival, although today most, which made it a double superlative, is usually omitted.
See also: cut, of, unkind