To dip for a wig. Formerly, in Middle Row, Holborn, wigs of different sorts were, it is said, put into a close-stool box, into which, for three-pence, any one might dip, or thrust in his hand, and take out the first wig he laid hold of; if he was dissatisfied with his prize, he might, on paying three halfpence, return it and dip again.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Dining Room Post * the DipNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes