A nick name for a soldier, from the colour of his clothes. To boil one’s lobster, for a churchman to become a soldier: lobsters, which are of a bluish black, being made red by boiling. I will not make a lobster kettle of my ****, a reply frequently made by the nymphs of the Point at Portsmouth, when requested by a soldier to grant him a favour.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
See also the definition in Nathan Bailey's 1736 dictionary of canting and thieving slang.
Lobscouse * LockNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes