Money, silver, prize. He is in for the plate; he has won the KEAT, i.e. is infected with the venereal disorder: a simile drawn from hofse-racing. When the plate fleet comes in; when money comes to hand.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
To Plant * Platter-facedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes