I have done a number of searches to determine why compounds of acids and bases are known as salts but came up empty. The fact that common salt dominates the word's usage means all my searches bring up the history of sodium chloride.
Is there a known origin of the word salt as applies to general compounds of acids and bases? Just like there is a well known origin of the word alkali.
The word salt had had different meanings in alchemy and ancient chemistry (> 1000 years ago). Some of the meanings are somewhat unexpected as you will see below. When new researchers say all their searches came up empty, it often means they are searching the free web. The fact is that most of the professional and high quality science is still behind paywalls.
Salt meant the following, selected from the unabridged Oxford dictionary (behind paywall):
Alchemy and Old Chemistry. One of the supposed ultimate elements of all substances. Obsolete.
Example: 1723 J. Clarke tr. Rohault's Syst. Nat. Philos. I. i. xx. 109 Hence they conclude, that these five Substances, viz. Mercury, Phlegm, Sulphur, Salt and Caput mortuum..are the only and the true Elements of all..mixed Bodies.
- Old Chemistry. A solid soluble non-inflammable substance having a taste. Obsolete.
Finally, the modern meaning appears from 1779...
Modern Chemistry. A compound formed by the union of an acid radical with a basic radical; an acid having the whole or part of its hydrogen replaced by a metal. (In wider theoretical use the term ‘salt’ includes acids as salts of hydrogen.) Also, ethereal salt, an ester.
The first marked step towards the modern conception of a chemical salt was Rouelle's definition (a1770) of a neutral salt as a compound formed by the union of an acid with any substance serving as a base for it and giving to it a concrete or solid form. Various modifications of this or earlier views were put forward until the publication of Lavoisier's definition of a salt as the union of an acid with an oxide; this definition, however, was found to be inadequate when the composition of the halogen compounds, sulphides, etc. came to be accurately known. A further revolution in the theory of salts was made by Berzelius, who divided them into two classes; viz. haloid adj. and n. salts, formed of an electropositive element and a halogen, and amphide adj. salts, resulting from the union of an acid and a base; the latter he subdivided into oxy-salts, sulpho-salts, selenio-salts, and telluri-salts. According to more recent conceptions (Arrhenius 1888) salts, including acids, are regarded as composed of positive ions or cations (hydrogen and metals) and negative ions or anions (halogens and acid radicals).
As a complement to M. Farooq explanations, I would add that in the beginning of the $19$th century a salt was a substance produced by the reaction of an acid on a hydroxyde, like the sea salt which can be obtained by the reaction $\ce{NaOH + HCl -> NaCl + H2O}$.
In general, acid and hydroxyde were always reacting according to the general equation : $$\ce{Acid + Hydroxyde -> Salt + Water}$$
The origin of the word ${\it sal}$ in chemistry is Latin and is associated in general with things from the earth. Usage of the term dates to the pseudo-Aristotle's ${\it De\ Perfecto\ Magisterio}$ of mediaeval time; and is also apparent in Arabic alchemy, where one sees ${\it sal\ ammoniac}$ appearing in reference to "the eagle."
There are 3 individuals named as contributing significantly to chemistry: Libavius, Agricola, and Rudolph Glauber. Glauber, for example, contributed his interpretation of the story of the Golden Fleece to include mention of "Sal Mirabile," which is actually "Glauber's salt."
This is the foundation of the answer to your question,
Is there a known origin of the word salt as applies to general compounds of acids and bases?
Keep in mind that the use of the word 'salt' (in any language, with respect to chemistry) was put forward by those who kept using it. So to a large extent, one can say that it's always been done that way. On the other hand, the systematic naming of salts as a chemical class - and not a name - comes later.
In this answer, I will be taking material from:
M.P. Crosland. ${\it Historical\ Studies\ in\ the\ Language\ of\ Chemistry}$. Dover Publications, New York. 1978.
I can provide the references he uses if requested.
The problem was introducing a systematic nomenclature for describing substances because there was an ever-increasing number of them being discovered.
The general historical evolution starts with the fields of botany and chemistry at the beginning of the 18th century, where Linnaeus' botanical binomial nomenclature was used by Bergman toward the end of the century. Guyton de Morveau extended and refined Bergman's scheme and collaborated with Lavoisier to establish what we work with today.
In the 16th century, Paracelsus referred to potassium bisulphate (which is more soluble than potassium sulphate) as ${\it sal\ enixum}$ in the alchemical literature. Paracelsus was famous (in part) for putting forth the theory that metals are composed of "Mercury, Sulphur and Salt," arising in alchemy.
Later, in 1664, the French chemist Le Fubure published work in which he differentiated salts of nitre by crystal habit. Duhamel, in 1736, showed that the salts of potassium and sodium were distinguishable by their bases.
In any case, there was a pressing need to formalize a nomenclature. Torbern Bergman (1735-84) is credited with taking on the task of reform in the first orderly manner. He introduced (via publication) his ideas towards a binomial system for naming, which was limited although a good start.
Guyton de Morveau expanded on Bergman's ideas and was able to systematically name 500 salts from 18 acids and 24 bases (earths, alkalis, and metals).
The big arrival was ${\it Méthode\ de\ nomenclature\ chimique}$ in 1787 and was authored by Guyton de Morveau, Lavoisier, Berthollet, and Fourcroy. It is at the point that the system for naming chemicals was in place. From here, the contributions of British, German, and other chemists was incorporated.
By the early 19th century, additions and revisions to the above were made and the systems of naming was largely in place. The evolution of naming things from the earth as 'salts' to naming things in a certain class as 'salts' largely took place during the last quarter of the 18th century.
