A hart: that animal is, in the Psalms, said to pant after the fresh water-brooks. Also the human heart, which frequently pants in time of danger. Cant.
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.
Panny * Pantile ShopNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes