late 15c., "sodium carbonate," an alkaline substance extracted from certain ashes (now made artificially), from Italian sida (or Medieval Latin soda) "a kind of saltwort," from which soda was obtained, of uncertain origin. Perhaps it is from a Catalan sosa, attested from late 13c., of uncertain origin. Proposed Arabic sources in a name of a variety of saltwort have 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 from ash produced by burning various seaside plants. A major trading commodity in the medieval Mediterranean, since commercial manufacture of it began in France in late 18c., these other 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.
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. typically flavored and sweetened with syrups. First record of soda pop is from 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; soda jerk first attested 1915 (soda-jerker is from 1883). Colloquial pronunciation "sody" is represented in print from 1900 (U.S. Midwestern).
"tedious and ineffectual person," 1935, American English carnival slang, of uncertain origin. Perhaps from jerkwater "petty, inferior, insignificant" [Barnhart, OED]; alternatively from, or influenced by, verbal phrase jerk off "masturbate" [Rawson]. The lyric in "Big Rock Candy Mountain," sometimes offered as evidence of earlier use, apparently is "Where they hung the Turk [not jerk] that invented work."
A soda-jerk (1915; soda-jerker is from 1883) is so called for the pulling motion required to work the taps.
The SODA-FOUNTAIN CLERK
Consider now the meek and humble soda-fountain clerk,
Who draweth off the moistened air with nimble turn and jerk,
[etc., Bulletin of Pharmacy, August, 1902]
apparently an early Middle English merger of three related Old English nouns, flota "boat, fleet," flote "troop, flock," flot "body of water, sea;" all from the source of float (v.). The early senses were the now-mostly-obsolete ones of the Old English words: "state of floating" (early 12c.), "swimming" (mid-13c.); "a fleet of ships; a company or troop" (c. 1300); "a stream, river" (early 14c.). From c. 1300 as an attachment for buoyancy on a fishing line or net; early 14c. as "raft." Meaning "platform on wheels used for displays in parades, etc." is from 1888, probably from earlier sense of "flat-bottomed boat" (1550s). As a type of fountain drink, by 1915.
Float.—An ade upon the top of which is floated a layer of grape juice, ginger ale, or in some cases a disher of fruit sherbet or ice cream. In the latter case it would be known as a "sherbet float" or an "ice-cream float." ["The Dispenser's Formulary: Or, Soda Water Guide," New York, 1915]
Few soda water dispensers know what is meant by a "Float Ice Cream Soda." This is not strange since the term is a coined one. By a "float ice cream soda" is meant a soda with the ice cream floating on top, thus making a most inviting appearance and impressing the customer that you are liberal with your ice cream, when you are not really giving any more than the fellow that mixes his ice cream "out of sight." [The Spatula, Boston, July, 1908]
c. 1400, "native sodium carbonate" (a sense now obsolete), from Old French nitre (13c.), from Latin nitrum, from Greek nitron, which is possibly of Eastern origin (compare Hebrew nether "carbonate of soda;" Egyptian ntr). Originally a word for native soda, but also associated since the Middle Ages with saltpeter (potassium nitrate) for obscure reasons; this became the predominant sense by late 16c.