Pureness comes from pure + -ness; pure derives from Old French pur, from Latin purus ('clean, unmixed, undefiled'). The historical chain includes Old French pur, Latin purus, and Proto-Indo-European *peuH-. The quoted older sense, "clean, unmixed, undefiled," supplies the immediate semantic anchor for pureness. The formation is best read as suffixal and borrowed: the word pureness carries its older form-history into a compact English lexical shape. In present English it is used as a noun, "the quality of being unmixed or untainted". Related English forms include pure, purity, purify, and purification. The result is a noun whose etymology preserves both the older form and the later English specialization.