A guinea pig is somebody or something that is used to test new ideas or products.
Guinea pigs are small stout-bodied rodents. In the UK they are often kept as much-loved pets. Unfortunately, they are also often used in research labs for research and biological experiments, and this is the origin of this idiomatic expression.
Examples of use:
1. I wanted to test a new cake recipe so I used my friend Dominique as a guinea pig.
2. They're looking for volunteers to act as guinea pigs to test their new perfume.
3. The students aren't happy to be acting as guinea pigs for your new English training.
4. My friend gets paid to be a research guinea pig for new medicines.
Can you use this expression in a sentence in the comments?
Image © Angela Boothroyd