A Guinea Fridge is an extra fridge found in many homes other than the kitchen - usually in the basement or garage - and which is older, not in very good shape, and often uses a ridiculous amount of electricity compared to newer appliances. It is used to hold beer for parties or for the huge amounts of left-over food from holiday meals, parties, or other social and/or family get-togethers (i.e. wakes). Its older and less reliable status means it is an experiment in trust to put food or even beer in it and then to trust that the food will be kept at a safely cold temperature so as not to spoil and poison you (or in the instance of beer, will have a temperature control that works well enough not to freeze, and therefore, skunk' your beer.) From the slang, "guinea pig", it involves experimentation or its use for experimentation upon by pharmaceutical or cosmetic companies or just in general. When empty, a Guinea Fridge is often a danger to small children or dim adults who may get locked in one and suffocate during a game of hide and seek.
There was such a mountain of left-over food from Grandma's wake that we had to put some of it in the Guinea Fridge in the basement. My brother ate some egg and ham salad for breakfast so we should know pretty soon whether the experiment was successful or if he'll have to go on a respirator until the Botulism Toxin leaves his system and he can breathe on his own again, so stay off the phone in case we have to call an ambulance. Eat any of the food in it at your own risk.