A term applicable to either the masculine or feminine gender, signifying a girl’s lover, or a man’s mistress: derived from a sweet cake in the shape of a heart.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Sweet * SweetnessNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes