solidness

late 14c., "not empty or hollow," from Old French solide "firm, dense, compact," from Latin solidus "firm, whole, undivided, entire," figuratively "sound, trustworthy, genuine," from PIE *sol-ido-, suffixed form of root *sol- "whole" (cf. Greek holos "whole," Latin salus "health," salvus "safe;" see safe (adj.)).

Meaning "firm, hard, compact" is from 1530s. Meaning "entirely of the same stuff" is from 1710. Of qualities, "well-established, considerable" c.1600. As a mere intensifier, 1830. Slang sense of "wonderful, remarkable" first attested 1920 among jazz musicians. As an adverb, "solidly, completely," 1650s. Solid South in U.S. political history is attested from 1858. Solid state as a term in physics is recorded from 1953; meaning "employing solid transistors (as opposed to vacuum tubes)" is from 1959. Related: Solidly.