means The quality of tasting like sugar, or by extension, a quality of being kind, pleasant, or lovable.

from From Old English "swetnes," built on "swete" (sweet), which traces back to a Proto-Germanic root "swotja-" and beyond that to a deep Proto-Indo-European "swad-" meaning pleasant or agreeable. That same ancient root is a cousin of Latin "suavis" (suave, agreeable) and "suadere" (to urge, persuade — literally to make a thing seem sweet), as well as Greek "hedys" (sweet, the heart of "hedonist"). The "-ness" is the plain old English suffix that turns a quality into a thing you can name.