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Compare Italian cognate giuncata "cream cheese-like dish" (so called because originally made or served on a bed of rushes); Middle English jonket also had this sense, which survived longer in dialects.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]