In the middle of an emotional speech at Caesar's funeral, Mark Antony tells the crowd that, of all the conspirators' knives, the one that hurt Caesar the most was wielded by Brutus:
For Brutus, as you know, was Caesar's angel:
Judge, O you gods, how dearly Caesar loved him!
This was the most unkindest cut of all.
The last line is an example of pleonasm, a word which comes from the Greek for "excessive." Pleonasm is the use of more words than are strictly necessary to get the meaning across; it is generally used as a rhetorical device for emphasis. A common example is the phrase "I saw it with my own eyes"; this is true of everything anyone has ever seen, but the pleonasm emphasizes the point that the speaker is certain of what they saw.
In this case, the pleonasm looks like a grammatical mistake, and the phrase is ungrammatical—intentionally so. Two different forms of the superlative combine ("most" and "unkindest") to emphasize just how unkind this cut was. Caesar, a conquering hero and a benefactor of the state, was not only being stabbed to death, but one of the men stabbing him was his closest friend, who betrayed their friendship as he murdered Caesar. This was not merely unkind, but "the most unkindest." The pleonasm here is typical of Antony's oratory in this scene, which is full of rhetorical devices used so skillfully that carefully constructed speeches appear to be outpourings of emotion.