Is the term "guinea pig" defined in the Pilot/Controller Glossary?
Being a "guinea pig" means to be the first to try out a (potentially dangerous) situation. There is no mention, past or current of the term "guinea pig" or any derivation of that.

Slang slips through on the radio, this is probably an instance of that. If the pilot or controller used that term the appropriate response would be "unclear, please say again". 

There is also no mention of it in the AIM.