derived from “Cucumber.” The joke is this: King Jeremiah = Jere'-king, contracted in Jer'-kin', or gher-kin,
and gherkin is a young cucumber.
The British Jeremiah.
Gibbon so calls Gildas, author of Lamentations over the
Destruction of Britain
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894