sexual tension
A tension, awkwardness, or underlying unease in the relationship of two people resulting from a strong mutual sexual attraction between them that has not been consummated. Just ask Chuck out already—I'm sick of being around you two and witnessing all your sexual tension. I can just feel the sexual tension every time Matt and I are together.
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.