Seriousness comes from serious (via Old French serieux from Late Latin seriosus, from Latin serius, 'weighty, grave') plus the noun-forming suffix -ness. The historical chain includes Latin serius. The quoted older sense, "weighty, grave," supplies the immediate semantic anchor for seriousness. The formation is best read as suffixal and borrowed: the word seriousness carries its older form-history into a compact English lexical shape. In present English it is used as a noun, "the quality of being grave, earnest, or of grave importance". Related English forms include serious and seriously. The result is a noun whose etymology preserves both the older form and the later English specialization.