(Stock Exchange term). A gentleman of sufficient name to form a bait who allows himself to be put on a directors list for the guinea and lunch provided for the board. (See Floaters.)
(A). A midshipman. A guineapig is neither a pig nor a native of Guinea; so a middy is neither a sailor nor an officer.
“He had a letter from the captain of the Indiaman, offering you a berth on board as guineapig, or midshipman.” —Captain Marryat: Poor Jack. chap. xxxi.
A special juryman who is paid a guinea a case: also a military officer assigned to some special duty, for which he receives a guinea a day, are sometimes so called.
(A), in the Anglican Church, is a clergyman without cure, who takes occasional duty for a guinea a sermon, besides his travelling expenses (second class) and his board, if required.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Guineapig from Infoplease: