solder v.

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.

c. 1300, souder, soudiour, "fighting man, one engaged ion military service," from Old French soudier, soldier and Anglo-French variants, "one who serves in the army for pay," from Medieval Latin soldarius "a soldier" (source also of Spanish soldado, Italian soldato), literally "one having pay," from Late Latin soldum, extended sense of accusative of Latin solidus, name of a Roman gold coin, properly "coin of thick or solid metal," not of thin plate (see solid (adj.)).

The -l- has been regular in English since mid-14c., in imitation of Latin. But the old pronunciation persisted in 16c.-17c. spelling variants sojar, soger, sojour; colloquial sojer appears in print in U.S. Civil War (Willie and Joe always say sojer in the Bill Mauldin World War II cartoons).

Modern French soldat is borrowed from Italian and displaced the older French word; one of many military (and other) terms French picked up during the Italian Wars in early 16c.; such as alert, arsenal, colonel, infantrie, sentinel.

As "one who obeys the commands of and contends in the cause of another," mid-14c., originally especially in the language of faith. Figurative uses 18c.-19c. tended toward notions of "armored," "combative," and, of things and animals, "reddish," for the uniform color. Of ants or termites that take on combat roles, by 1781.

Old slang names for military men circa early 19c. include mud-crusher "infantryman," cat-shooter "volunteer," fly-slicer "cavalryman," jolly gravel-grinder "marine." Soldier boy is attested from 1861. Old soldier "one practiced or experienced" in anything is by 1722.

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."