Don’t lose a sheep for a halfpennyworth of tar: tar is used to mark sheep. A jack tar; a sailor.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Taplash * TaradiddleNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes