you could cut (something) with a knife
Something intangible or non-solid is incredibly thick, palpable, or poignant. The air was so humid as we stepped off the plane if felt like you could cut it with a knife. The entire dinner was so awkward you could cut the tension with a knife.
you could cut it with a knife
Alluding to something very thick, such as muggy air or a heavy accent; also, a very tense atmosphere. For example, The smoke was so thick you could cut it with a knife, or When I walked in they all stopped talking and you could cut the air with a knife. [Colloquial; late 1800s]