unboiled lobster, n. — Green’s Dictionary of Slang

unboiled lobster n.

[the uncooked lobster is blue, like a police uniform. The cooked lobster has turned red/pink, like a soldier’s]

a policeman.

1829		J.B. Buckstone Billy Taylor I iii: car.: [I] am no more a dull drab-coated watchman. [...] mary: Thou unboiled lobster, hence.
1829		Sussex Advertiser 21 Dec. 4/3: Two new policemen, profanely called ‘unboiled lobsters,’ showed their ‘horrid fronts’.
c.1840		‘The Wonders of the Age’ in Holloway & Black II (1979) 226: You can’t out of doors your nob stir, / Without being watch’d by an unboil’d lobster.
1843		Nothern Star 22 Apr. 1/3: The nature of which was the calling of police unboiled or raw lobsters.
1856		W. Middlesex Advertiser 8 Nov. 4/4: From the highest offices to the ‘unboiled lobster,’ there is Paddy to be found.
1860		Hotten Dict. of Modern Sl. etc. (2nd edn).
1864, 1867, 1870		Hotten Sl. Dict.
1873		Sl. Dict.
1884		Ally Sloper’s Half Holiday 7 June 47/2: To others Samuel Hardstaff is a peeler, a reeler, a copper, a Bobby, a Robert, an unboiled lobster, or a slop, but to cook he is Mr Policeman.