navigator, n. — Green’s Dictionary of Slang

navigator n.

also nav

[rhy. sl.; the connection with the predominantly Irish navigators, builders of Victorian Britain’s railways and canals, whose stereotype consumed many potatoes, may or may not be coincidental]

a potato.

1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1893		P.H. Emerson Signor Lippo XIV As we were dining, in came North Eye carrying a dish from the bake-house, a sheep’s knock over a dollop of navs.
1914		E. Wittmann ‘Clipped Words’ in DN IV:ii 124: nav, from navigator. A potato.
2011		Spitalfields Life 18 Nov. 🌐 ‘Navigators’ is taters.

In compounds

navigator scot (n.) [ext. of rhy. sl. navigators = Cockney pron. of (po)taturs + scot = hot; ult. Antarctic explorer Robert Falcon Scott (1868–1912)]

baked potatoes .

1859		Hotten Dict. of Modern Sl. etc. 144: NAVIGATORS, taturs [...] NAVIGATOR SCOT, baked potatoes all hot.
1928		Bath Chron. 4 Aug. 25/6: An enthusiast arrived [...] to quote: [...] ‘navigator Scot,’ baked potatoes all hot; ‘Joe Savage,’ a cabbage; ‘bowl the hoop,’ soup; ‘Billy Button,’ mutton.