Green’s Dictionary of Slang

foredeck n.

the vagina.

1650		H. Nevile Newes from the New-Exchange 2: This is a Lady indeed, that seaven years since took sayle with Presbytery, being chargd in the Fore-deck by Master Hollis, in the Poop by Master Pim, whilst she clapt my Lord of Holland under hatches.
c.1927		in P. Smith Letter from My Father (1978) 426: He would lap up his wife’s wet deck and then ‘frig’ her himself.