The term "guinea pig" is an idiomatic expression that refers to a person or thing that is used as an experimental subject, often without their knowledge or consent. The phrase originated in the 17th century when scientists first began using guinea pigs for medical research due to their small size and ease of handling. Over time, the term has come to be used more broadly to refer to anyone who is being tested, tried, or experimented on, often without their consent or knowledge.