solidsolid:  Solid comes via Old French solide from Latin solidus ‘solid, whole’ (source also of English solder and soldier and of the French coin term sou). It went back to the same base (Indo-European *sol-) that produced Latin sollus ‘whole’ (source of English solemn and solicit) and salvus ‘unharmed’ (source of English safe, save, etc).
solid (adj.)late 14c., "not empty or hollow," from Old French "firm, dense, compact," from Latin "firm, whole, undivided, entire," figuratively "sound, trustworthy, genuine," from PIE , suffixed form of root "whole" (cognates: Greek "whole," Latin "health," "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. in U.S. political history is attested from 1858. 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: .solid (n.)late 14c., "three-dimensional figure," from solid (adj.). Meaning "a solid substance" is from 1690s. Compare also solidus; Latin (adj.) was used as a noun meaning "an entire sum; a solid body."