Samuel Foote’s comedyThe Nabob, now obscure, is the first linkage of picnic with the euphemism “nick-nack.” He used in the sense of dining en piquenique, which suggests familiarity. The alliterative corruption is meant to be humorous for those in the know of the trendy Parisian custom:

Janus Time enough. —You had no particular commands, master Conserve?
Conserve Only to let you know that Betsy Robins has a rout and supper on Sunday next.
Janus Constant still, Mr. Conserve, I see. I am afraid I can’t come to cards, but shall be sure to attend the repast. A nick-nack, I suppose?
Conserve Yes, yes; we all contribute, as usual: The substantials from Alderman Sirloin’s; Lord Frippery’s cook finds fricassees and ragouts; Sir Robert Bumper’s butler is to send in the wine, and I shall supply the desert.
Janus. There are a brace of birds and a hare that I cribbed this morning out of a basket of game.
Conserve They will be welcome.   [Act 1, sc. 2]

A nick-nack is a meal to which guests dined are expected to bring a share. Where Foote got nick-nack from is a mystery. Perhaps he coined it?

Featured Image: Attributed to Joseph Wright of Derby. Samuel Foote (left) David Garrick and his wife Eva Maria Veigel (with her exposed breasts)

See  Foote, Samuel. The Nabob; a Comedy in Three Acts. London, 1772