Put/stick the knife in Definition 1 of 2Pro Tip 1/3Emotional Charge Used when feelings are involved; it suggests betrayal has an emotional impact. She put the knife in by mocking my dreams in front of our friends. Pro Tip 2/3Friendship's End Often used when a friend acts disloyally, terminating the trust in a relationship. He really stuck the knife in when he cheated with my girlfriend. Pro Tip 3/3Unexpected Hurt Implies that the hurtful act was surprising and not anticipated. I never expected her to put the knife in by revealing my secrets.