Physical herbs; also follies. He must go to Battersea, to be cut for the simples—Battersea is a place famous for its garden grounds, some of which were formerly appropriated to the growing of simples for apothecaries, who at a certain season used to go down to select their stock for the ensuing year, at which time the gardeners were said to cut their simples; whence it became a popular joke to advise young people to go to Battersea, at that time, to have their simples cut, or to be cut for the simples.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Simpleton * To SingNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes