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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.
solid sol·id (sŏl'ĭd)
Of definite shape and volume; not liquid or gaseous.
Firm or compact in substance.
Having no internal cavity or hollow.
A solid substance, body, or tissue.
Food that is relatively firm in substance or that must be chewed before swallowing.
To persist doggedly: A little warning bell went off, but I soldiered on (1954+)