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.