1748 (in Chesterfield's "Letters"), but the thing itself apparently was rare before c. 1800 as an English institution [OED]; it originally meant "a fashionable social affair (not necessarily out of doors) in which every partaker contributed something to the general table;" from French piquenique (1690s), perhaps a reduplication of piquer "to pick, peck," from Old French (see pike (n.1)), or the second element may be nique "worthless thing," from a Germanic source.
As in many other riming names, the elements are used without precision, but the lit. sense is appar. 'a picking or nibbling of bits,' a snatch, snack .... [Century Dictionary]
The word also turns up 18c. in German, Danish, Swedish. Later "pleasure party the members of which carry provisions with them on an excursion, as to some place in the country." Figurative sense of "something easy" is from 1886. Picnic basket is by 1857. Picnic table is by 1858, originally a folding table used for outdoor dining.
Compare Italian cognate giuncata "cream cheese-like dish" (so called because originally made or served on a bed of rushes); Middle English jonket also had this sense, which survived longer in dialects. Johnson (1755) also records a verb junket "to feast secretly; to make entertainments by stealth."
Old English wæt "moist, rainy, liquid," also as a noun. "moisture, liquid drink," from Proto-Germanic *wed- (source also of Old Frisian wet ). Also from cognate Old Norse vatr; all from PIE root *wed- (1) "water; wet." Of paint, ink, etc., "not yet dry" from 1510s. Opposed to dry in reference to the U.S. battles over prohibition from 1870. Wet blanket "person who has a dispiriting effect" is recorded from 1871, from use of blankets drenched in water to smother fires (the phrase is attested in this literal sense from 1660s).
Do we not know them, those wet blankets who come down on our pleasant little fires and extinguish them, with no more ruth than the rain feels when it pours on the encampment of the merry picnic party, or floods the tents of a flower show? ["Wet Blankets," in Godey's Lady's Book and Magazine, February, 1871]
All wet "in the wrong" is recorded from 1923, American English; earlier simply wet "ineffectual," and perhaps ultimately from slang meaning "drunken" (c. 1700). Wet-nurse is from 1610s. The diver's wet-suit is from 1955. Wet dream is from 1851; in the same sense Middle English had ludificacioun "an erotic dream."
He knew som tyme a man of religion, þat gaff hym gretelie vnto chastitie bothe of his harte & of his body noghtwithstondyng he was tempid with grete ludificacions on þe nyght. ["Alphabet of Tales," c. 1450]
"metal vessel used for boiling or heating liquids over a flame," Old English cetil, citel (Mercian), from Proto-Germanic *katilaz (compare Old Saxon ketel, Old Frisian zetel, Middle Dutch ketel, Old High German kezzil, German Kessel), which usually is said to be derived from Latin catillus "deep pan or dish for cooking," a diminutive of catinus "deep vessel, bowl, dish, pot," from Proto-Italic *katino-.
This word has been connected with Greek forms such as [kotylē] "bowl, dish." Yet the Greek word is no perfect formal match, and words for types of vessels are very often loanwords. It seems best to assume this for catinus too. [de Vaan]
One of the few Latin loan-words in Proto-Germanic, along with *punda- "measure of weight or money" (see pound (n.1)) and a word relating to "merchant" that yielded cheap (adj.). "[I]t is striking that all have something to do with trade" [Don Ringe, "From Proto-Indo-European to Proto-Germanic," Oxford 2006]. Perhaps the Latin word was confused with a native Germanic one.
Spelling with a -k- (c. 1300) probably is from influence of Old Norse cognate ketill. The smaller sense of "tea-kettle" is attested by 1769.
Kettle of fish "complicated and bungled affair" (1715), sometimes is said to be from a Scottish custom of a kettle full of fish cooked al fresco at a boating party or picnic, but this custom is not attested by that phrase until 1790. Perhaps it is rather a variant of kittle/kiddle "weir or fence with nets set in rivers or along seacoasts for catching fish" (c. 1200, in the Magna Charta as Anglo-Latin kidellus), from Old French quidel, probably from Breton kidel "a net at the mouth of a stream."
Kettle was used in geology for "deep circular hollow in a river bed or other eroded area, pothole" (1866), hence kettle moraine (1883), one characterized by such features.