These are well known to crews of British merchant navy ships and North Sea oil rigs. They’re pastries or cakes, always baked on board, that are usually served at tea breaks, morning or afternoon.
The first known user of the term in print is Malcolm Lowry. In his Under the Volcano of 1947 appears: “The tabnabs were delicate and delicious little cakes made by the second cook.” He wrote even earlier:
It was one bell, a quarter to four, time to make the tea for the crew. ... He filled the teapot which he had left in the galley since dawn and took it with the tabnabs down aft to the forecastle.
Ultramarine, by Malcolm Lowry, 1933. It is certain that Lowry got this term from personal experience, as this novel is based on his experience of shipping as a deckboy on a Liverpool freighter to Yokohama in 1928.
It is definitely older. An indication of that is an old sea song:
Now come wid me, me dearie,
An’ I will stand ye treat,
I’ll buy ye rum and brandy, dear,
An’ tabnabs for to eat.
The New York Gals, reproduced in Shanties from the Seven Seas, by Stan Hugill, 1961. The date is unknown but Mr Hugill says it goes back before the 1930s.
Its provenance is obscure. It may originally have been Royal Navy slang, but every written reference is to merchant shipping. An editor’s footnote to The Collected Poetry of Malcolm Lowry (1992) says they were “small pasty delicacies, usually reserved for officers and passengers (thus, to be nabbed from the table).” The suggested origin is plausible, but unverified (and probably unverifiable).
There certainly seems to have been a class-consciousness about them at one time. Wilfred Granville defined them in his Sea Slang of the Twentieth Century (1950) as a steward’s term in the merchant navy for “Buns, pastry and confectionery which are reserved for the passengers in a liner and are not for the crew.” The term is certainly more democratic than that today, and perhaps always was, away from the stifling conventions of cruise ships and liners.
They may not always have been fancy cakes, either. Reader Mike Harrison wrote from Canada: “My father was a merchant mariner in the catering department in the 1930s. During my British childhood in the 1940s and 1950s in Southampton, tabnabs was common in our house and it seemed to have currency throughout the city. It was a loose term but described small and simple cupcakes and biscuits for day-to-day snacks. When company was expected something more elaborate was baked.”
The term is still in use, though it doesn’t often reach searchable printed sources (a mobile catering van in Orkney, however, is called Tea and Tabnabs, a frequent reference to the afternoon break). This is a rare recent appearance in fiction:
If Keith didn’t call in to see the baker during the morning the baker would go and find Keith to ask what tabnabs he would like. Keith’s favourites at the time were meringues and, whatever else the baker made, there were always two waiting for him at afternoon break.
Zizzeddu, by Peter K Thiot, 2007.