Loquacious, not able to keep a secret. He is as long-tongued as Granny: Granny was an idiot who could lick her own eye. See Granny.
Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose.
Long Stomach * Long-windedNathan Bailey's 1736 Dictionary of canting and thieving slang
John S. Farmer's collection of canting songs and slang rhymes