KNIFE

• knife plural knives [countable]
• 1 a metal blade fixed into a handle, used for cutting or as a weapon [↪ scalpel]:
• a knife and fork
• Some young people are carrying knives to defend themselves.
• a kitchen knife
• Use a sharp knife to cut the melon into sections.
• 2

   the knives are out (for somebody)

  informal used to say that people are being extremely unfriendly in criticizing someone:
• The knives are out for the vice president.
• 3

   twist/turn the knife (in the wound)

  to say something that makes someone more upset about a subject they are already unhappy about
• 4

   stick/put etc the knife in/into someone

  British Englishinformal to dislike someone and be very unfriendly towards them
• 5

   under the knife

  informal having a medical operation
• 6

   you could cut the atmosphere/air/tension with a knife

  used to say that you felt the people in a room were angry with each other
• 7

   like a (hot) knife through butter

  informal used to say that something happens or is done very easily, without any problems

Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.