solidity (n.)

late 14c., "not empty or hollow, hardened;" of figures or bodies, "having three dimensions," from Old French solide "firm, dense, compact," from Latin solidus "firm, whole, undivided, entire," figuratively "sound, trustworthy, genuine," from suffixed form of PIE root *sol- "whole."

The meaning "firm, hard, compact" is from 1530s. Of arguments, etc., "substantial" (opposed to frivolous or flimsy). The meaning "entirely of the same stuff" is from 1710. Of qualities, "well-established, considerable" c. 1600. Of food from c. 1700.

As a mere intensifier, "thoroughly, downright," by 1830. The slang sense of "wonderful, remarkable" is attested by 1920 among jazz musicians.

As an adverb, "solidly, completely," 1650s. Solid South in U.S. political history is attested from 1858 on the notion of unanimity in voting; solid in this sense (in reference to New York) is by 1855. Solid state as a term in physics is recorded from 1953; the meaning "employing printed circuits and solid transistors" (as opposed to wires and vacuum tubes) is from 1959.

also solə-, Proto-Indo-European root meaning "whole, well-kept." 

It forms all or part of: catholic; consolidate; consolidation; holism; holo-; holocaust; Holocene; hologram; holograph; insouciant; safe; safety; sage (n.1) kind of herb; salubrious; salutary; salute; salvage; salvific; salvo "simultaneous discharge of guns;" save (v.) "deliver from danger;" save (prep.) "except;" solder; soldier; solemn; solicit; solicitous; solid; solidarity; solidity; sou.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit sarvah "uninjured, intact, whole;" Avestan haurva- "uninjured, intact;" Old Persian haruva-; Greek holos "whole;" Latin salvus "uninjured, in good health, safe," salus "good health," solidus "solid;" Armenian olj "whole, healthy."