Being Anglophile and aware of London happenings, Irving probably picked up the aftermath of the Pic Nic Society scandal during his tour of Europe 1804-1806. The word stuck, but it’s used only once as an adjective to mean something silly. Under the heading “Fashions by Anthony Evergreen,” (1807) for the journal Salmagundi; or, The Whim-whams and Opinions of Launcelot Langstaff, Esq., and Others, they write “Picnic silk stockings, with lace clocks, flesh-colored are most fashionable, as they have the appearance of bare legs—. The stockings are carelessly bespattered with mud, to agree with the gown, which should be bordered about three inches deep with the most fashionable colored mud that can be found.”
Salmagundi, the name chosen for Irving’s magazine, is salami, a mix of ground meats, or in this instance, a miscellany.
Salmagundi’s picnic satire is itself satirized by Cole Porter in the lyrics to “Anything Goes” (1934):
In olden days a glimpse of stocking
Was looked on as something shocking,
But now, God knows,
Anything Goes.
See Washington Irving and James Kirke Paulding. Salmagundi; or, the Whim-Whams and Opinions of Launcelot Langstaff, Esq., and Others (1808). In The Complete Works of Washington Irving, edited by Bruce Granger and Martha Hartzog eds., vol 6. New York: Twayne, 1977.