Someone who gets experimented on first, or volunteers to be the test run for a new idea, product, plan, or dodgy recipe. If you say I’ll be the guinea pig, you’re offering to go first so everyone else can see if it’s brilliant or a complete car crash. Honestly, it’s brave or foolish depending on the snacks involved.
"They’ve bought some weird new energy drink from the corner shop and want a review. I sighed and went, fine, I’ll be the guinea pig. Ten minutes later I’m sweating like a kettle."