Green’s Dictionary of Slang

1. a general term of opprobrium; also attrib.

1748		Smollett Roderick Random (1979) 140: A good seaman he is [...] a brave fellow as ever crackt biscuit; none of your Guinea pigs.
1751		Smollett Peregrine Pickle (1964) 9: For my own part, d’ye see, I was none of your Guinea-pigs.
c.1790		‘Bad Half-pence’ in Holloway & Black II (1979) 241: You Mr. Burncrust and your proud guinea pig wife ought to be set in the pillory.
1795		A Puff at the Guinea Pigs n.p.: Both Lords and Ladies too, or wear they hair or wigs, sir, / If they throw flour on their heads, are called guinea pigs, sir.
1885		Bulletin (Sydney) 23 May 6/1: In the telegram concerning the ‘feed’ given to the Governor-elect of New South Wales, Lord Carington, by Saul Samuel, it is stated that that patronising ‘guinea-pig,’ ‘the Duke of Manchester, predicted that the colonies would, in the course of time, surpass Britain.’ – again, I am not sure what.

2. anyone whose fee comes to one guinea, e.g. a doctor.

1821		W. Combe Doctor Syntax, Wife (1868) 327/1: ‘Oh! Oh!’ cried Pat, ‘how my hand itches, / Thou guinea pig in boots and britches, / To trounce thee well’ [...] (Glanders) Just utter’d, ‘Here is my account, / And now I beg the small amount’.
1873		Sl. Dict. 184: Guinea pigs habitual directors of public companies; special jurymen; and engineer officers doing civil duty at the War Office, and paid a guinea per diem.

3. (UK Und.) a man who receives a guinea for talking up a second-rate horse.

1835		G. Kent Modern Flash Dict. 16: Guinea pig – a fellow who receives a guinea for puffing off an unsound horse.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].

4. anyone working only part-time, e.g. a company director who only attends board-meetings, a clergyman serving as a deputy.

1871		Temple Bar XXXI 320: [...] guinea pigs, [...] those gentlemen of more rank than means, who hire themselves out as directors of public companies, and who have a guinea and a copious lunch when they attend board meetings [F&H].
1883		Church Rev. 2 Jan. n.p.: Guinea pigs... are [...] unattached or roving parsons, who will take any brother cleric’s duty for the moderate remuneration of one guinea [F&H].
1931		A. Bennett Teresa of Watling Street 13: Lord Dolmer was poor – for a lord – and eked out a bare competence by being a guinea-pig in the City.

5. (US Und.) an informer, a stool pigeon.

1983		R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].
1994		M. Montague Dread Culture 131: Ah guess mi is di guinea pig. So dem charge yuh too?