Green’s Dictionary of Slang

a printer, i.e. typographer.

1827		Pierce Egan’s Life in London 18 Feb. 861/2: [A] pugilistic encounter took place [...] between a Typo and a Kill-bull [...] ‘The Roast Beef of old England’ was the motto of the butcher; and ‘The Freedom of Reporting, and Liberty of the Press’ that of the printer.
1833		Satirist (London) 7 Apr. 533/3: I have just been talking with Death,’ said typo Taylor to Figgins of Farringdon-street .
1871		Sportsman (London) ‘Notes on News’ 28 Oct. 4/2: A machine has been invented which not only sets types well, but as nearly possible thinks. Think of this, gentle typo, and, in the language of the poet, ‘haul up your slack’.
1895		Crowe Aus. Sl. Dict. 90: Typo, a printer.