This quote from Julius Caesar Act 3, Scene II is spoken by Marc Antony in his speech to the multitudes following Caesar's murder. In it, Antony is speaking of the traitor Brutus:
For Brutus, as you know, was Caesar's angel.
Judge, O you gods, how dearly Caesar lov'd him!
Brutus, as Antony says, was greatly beloved of Caesar. Therefore, that Brutus turned upon Caesar with murder was more unkind, unloving, than for others of the traitors to do so. Moreover, it went against the ways of what was natural between two individuals who value each other and profess to know each other's thoughts and motives as beloved friends.
As Shakespeare does so well, he is employing a double meaning for the words "unkind" and "cut." In the first, Shakespeare calls upon the two meanings of "unkind" as cruel and unnatural, as covered above. In the second, "cut" means both a wound, such as a murdering stab to Caesar's flesh, and as an act that separates people and severely wounds a person's feelings. Antony is implying that Caesar was not only murdered by Brutus's cut to his body but also by Brutus's cruel cut to his soul and emotional heart. In other words, Brutus killed Caesar two ways, by cutting his flesh with a knife and by betraying their friendship for and devotion to one another: "Et tu, Brute? Then fall, Caesar!" (Act 3, Scene I): [paraphrase: If even Brutus turns against me, then there is no reason to live.]
In addition to the above double plays on words, there is still one more double play. Shakespeare intentionally chose for Antony to say "most unkindest." In contemporary grammar usagae "most unkindest" is a redundancy: "most" and "-est" express the same superlative comparison; both make something as great as it can be. When Shakespeare combines them, and it is said that such a combination was a common usage in Elizabethan England, he is giving an additional double meaning to Antony's words: unkind is doubled in meaning (cruel and unnatural); cut is doubled (wound, severed friendship and feelings); and most is doubled by -est marking an empahsis to all Antony is saying.