This is one of those words you just avoid using, as an American, because it’s an insult to persons of Italian descent in the United States. That may be due to an association with some of the trivia related in this article?
https://www.urbandictionary.com/define.php?term=guinea&defid...
The article mentions 'guinea men' to refer to the trading ships that shuttled between Britain, South America and 'Guinea', but I can't help but imagine a hypothetical breed of six feet tall, dapperly clothed, bipedal guinea pig.