Solidness comes from solid + -ness. Solid derives from Old French solide, from Latin solidus ('firm, dense, whole'), from Proto-Indo-European *sol-ido-, from the root *sol- ('whole'). The historical chain includes Latin solidus and Proto-Indo-European *sol-. The quoted older senses, "firm, dense, whole" and "whole", supply the semantic anchors for solidness. The formation is best read as suffixal and borrowed: the word solidness carries its older form-history into a compact English lexical shape. In present English it is used as a noun, "the quality of being firm, dense, or dependable; solidity". Related English forms include solid, solidify, and solidity. The result is a noun whose etymology preserves both the older form and the later English specialization.