1A tailless South American rodent of the cavy family. Originally raised for food, it no longer occurs in the wild and is now typically kept as a pet or for laboratory research.
2A person or thing used as a subject for experiment.
Mid 17th century: from Guinea + pig; the term Guinea was probably a designation for an unknown distant country. Use in guinea pig (sense 2) arose in the 1920s in reference to the animal's use in laboratory research.