Green’s Dictionary of Slang

1. a cookshop under Furnival’s Inn, London, popular among legal clerks.

c.1786		Grose Classical Dict. of the Vulgar Tongue ms. additions n.p.: The Dip. A Cook’s Shop, under Furnival’s Inn, where many Attornies Clerks, and Inferior Limbs of the Law, take out the Wrinkles from their Bellys.
1788, 1796		Grose Classical Dict. of the Vulgar Tongue (2nd, 3rd edn) .

2. (UK gay) a stretch of Piccadilly adjoining Green Park, where gay prostitutes solicit wealthy clients.

1977

D. Powis Signs of Crime 181: Dip, the That part of Piccadilly (the thoroughfare) adjoining St James’s Park [sic], where male prostitutes once importuned wealthy homosexuals. A term known to many homosexuals throughout the UK and even abroad.