Source, in full
Unattributed in Punch, vol. 29, no. 19 (14 July 1855) p. 19. Attributed to Key "on the authority of F. J. Furnivall" in J. K. Hoyt, The Cyclopedia of Practical Quotations, rev. ed. (1896) p. 385. Authorship disputed by M. H. Spielmann, The History of "Punch" (London: Cassell & Co, 1895) p. 160
Notes from Wikiquote
"This well-known definition," says Dr. Furnivall, "according to the 'Academy,' was by Professor T. Hewitt Key; he sent it to Punch, and of course it was printed forthwith—I suppose, somewhere about the 'Sixties." But as a matter of fact this mot, which has also been attributed to Kenny, had already been published in "The Month" as early as August, 1851 (page 147, Vol. I); and I may add that though I remember hearing Professor Key quote it more than once, I never heard him pretend to its authorship.
Variants
in Harper's New Monthly Magazine, vol. 10, no. 56 (January 1855) p. 275. See under Anonymous#19th century