Unkindest cut of all
The exact quotation from Shakespeare’s Julius Caesar Act III, Scene II is “This was the most unkindest cut of all” which Marc Anthony uses to describe Brutus’s stabbing of Caesar in the figurative sense of a betrayal rather than the dagger thrust itself. In using Shakespeare’s expression, the word ‘most’ is usually omitted because ‘most’ unkindest sounds ungrammatical to modern ears, but this was not so in Shakespeare’s time.