Old English 1st & 3rd person singular present indicative of cunnan "to know," less commonly as an auxiliary, "to have power to, to be able," (also "to have carnal knowledge"), from Proto-Germanic *kunnjanan "to be mentally able, to have learned" (source also of Old Norse kenna "to become acquainted, try," Old Frisian kanna "to recognize, admit, know," German kennen "to know," Middle Dutch kennen "to know," Gothic kannjan "to make known"), from PIE root *gno- "to know."
It holds now only the third sense of "to know," that of "to know how to do something" (as opposed to "to know as a fact" and "to be acquainted with" something or someone). Also used in the sense of may, denoting mere permission. An Old English preterite-present verb, its original past participle, couth, survived only in negation (see uncouth), but compare could. The present participle has spun off with a deflected sense as cunning.
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).
Modern sense of "air-tight vessel of tinned iron" is from 1867. Slang meaning "toilet" is c. 1900, said to be a shortening of piss-can; meaning "buttocks" is from c. 1910, perhaps extended from this.