The adjective “acute” has multiple usages: describing something experienced to an intense degree, or describing a highly developed insight, a sharp point, or an angle less than 90 degrees. It makes sense, then, that “acute” was borrowed by photographers in the mid-20th century to describe the sharpness of an image. “Acutance” refers to the contrast between transitions of light and dark and how sharp those borders are. It doesn’t have anything to do with the resolution or pixels, which can appear to impact sharpness of digital images.