you could cut the atmosphere with a knife

• you could cut the air with a knife

The image is that the atmosphere is thick. Compare French à couper au couteau.

you could cut the atmosphere with a knife

1. Used to describe a situation with intense anger in the area.

• “you could cut the atmosphere with a knife”, in Cambridge English Dictionary, Cambridge, Cambridgeshire: Cambridge University Press, 1999–present.
• “you could cut the atmosphere with a knife” in Longman Dictionary of Contemporary English, Longman.
• “you could cut the air atmosphere with a knife” (US) / “you could cut the air atmosphere with a knife” (UK) in Macmillan English Dictionary.
• “you could cut it with a knife”, in Merriam-Webster Online Dictionary, Springfield, Mass.: Merriam-Webster, 1996–present.