late 15c., sode, "sodium carbonate, an alkaline substance extracted from certain ashes" (now made artificially), from Italian sida (or Medieval Latin soda), in reference to a kind of saltwort from which soda was obtained, a word of uncertain origin.
Perhaps it is from a Catalan sosa, attested from late 13c., also a word of uncertain origin. An Arabic name for a variety of saltwort has been proposed as the source, but this word has not been attested and that theory is no longer considered valid. Another theory, considered far-fetched in some quarters, traces it to Medieval Latin sodanum "a headache remedy," ultimately from Arabic suda "splitting headache."
Soda is found naturally in alkaline lakes, in deposits where such lakes have dried, and in ash produced by burning various seaside plants. It was a major trading commodity in the medieval Mediterranean, but since commercial manufacture of it commenced in France late 18c., natural sources have been abandoned. Washing soda (sodium carbonate) is commonly distinguished from baking soda (sodium bicarbonate). A soda-cracker (1863) has baking soda as an ingredient, as does soda bread (1850).
The meaning "carbonated water" is first recorded 1834, a shortening of soda water (1802) "water into which carbonic acid has been forced under pressure." "It rarely contains soda in any form; but the name originally applied when sodium carbonate was contained in it has been retained" [Century Dictionary, 1902]. Since 19c. they are typically flavored and sweetened with syrups.
The first record of soda pop is by 1863, and the most frequent modern use of the word is as a shortening of this or other terms for "flavored, sweetened soda water." Compare pop (n.1). Soda fountain is from 1824 for a metal structure that can dispense carbonated water; soda jerk for the operator of one is attested by 1915, so called for the action involved in drawing it (soda-jerker is from 1883; soda-fountain boy is from 1876). Colloquial pronunciation "sody" is represented in print from 1900 (U.S. Midwestern).