solidarity (n.)

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."

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 printed circuits and solid transistors" (as opposed to wires and vacuum tubes) is from 1959. Related: Solidly.

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."