The most unkindest cut of all
For Brutus, as you know, was Caesar's angel.
Judge, O you gods, how dearly Caesar lov'd him!
This was the most unkindest cut of all;
For when the noble Caesar saw him stab,
Ingratitude, more strong than traitors' arms,
Quite vanquish'd him: then burst his mighty
heart. . . .
Marc Antony is in the middle of his great speech on the assassination of Julius Caeser [see FRIENDS, ROMANS, COUNTRYMEN, LEND ME YOUR EARS]—which has very quickly turned into a character assassination of Brutus, a prominent republican, Caesar's friend and one of Caesar's murderers.
When Antony calls Brutus's stabbing of Caesar "the most unkindest cut of all," he is playing on two senses of "unkind"—"inhumane" and "unnatural." According to Antony, when Brutus literally "cut" the loving Caesar, a bloody deed was compounded with ingratitude. It wasn't the wound that killed Caesar, says Antony, but Brutus's treachery.
Time has softened Antony's language: all that remain are a weaker sense of "unkind" and a less literal use of "cut." "Most unkindest," by the way, wasn't as ungrammatical in Shakespeare's day as it is in ours.