From the second edition (1989):
tiffin, n.
(ˈtɪfɪn) Also 9 tiffing. [Appears to have originated in the Eng. colloq. or slang tiffing, vbl. n. from tiff v.2 to take a little drink or sip (cf. quot. 1785), which has been specialized in Anglo-Indian use.

1785 Grose Dict. Vulg. Tongue, Tiffing, eating, or drinking out of meal time. 1867 Wedgwood Dict. Eng. Etymol., Tiffin, now naturalised among Anglo-Indians in the sense of luncheon, is the North country tiffing (properly sipping).]

a. In India and neighbouring eastern countries, A light midday meal; luncheon.

1800 Ward in Carey's Life vi. (1885) 137 Krishna came to eat tiffin (what in England is called luncheon) with us. 1803 [see tiff v.4]. 1810 T. Williamson E. Ind. Vade M. I. 352 The [Mahommedan] ladies, like ours, indulge in tiffings (slight repasts). c1816 Mrs. Sherwood Stories on Ch. Catech. xvi. 141 She gave them a good tiffing about one o'clock. 1831 E. J. Trelawny Adv. Younger Son II. 115 When the gong sounds one, you will find tiffin in the hall. 1896 ‘H. S. Merriman’ Flotsam xx, I'll call for you after tiffin. 1906 Peking & Tientsin Times 9 May 1/2 Those wishing to have tiffins at the forthcoming spring meeting will please apply at the secretary's office. Price $2.00 per tiffin.

b. attrib., as tiffin-bell, tiffin-table, tiffin-time. tiffin-carrier, a tiered container for transporting meals.

1811 Mrs. Sherwood Henry & Bearer 31 The tiffin time was very stupid to the little boy. 1852 Life in Bombay 34 The preparation of the tiffin table. 1890 Clark Russell Shipmate Louise vi, The tiffin-bell rang. 1960 R. P. Jhabvala Householder i. 13 He always brought his breakfast with him in a tin tiffin-carrier.

Hence ˈtiffin v., (a) intr. to take tiffin, to lunch; cf. tiff v.4; (b) trans. to provide with tiffin.

1866 M. E. Braddon Lady's Mile xi, I'd tiffin them if they were my visitors. 1880 P. Gillmore On Duty 51 Here I tiffined. 1903 Ld. R. Gower Rec. & Remin. 388 We tiffined at a tea-house in the village.