The word "guinea" (also "guinny" or "ghinny") is a derogatory slang word for Italians or those of Italian descent.
The exact origin is not clear, but it may refer to the willingness of Italians to work for one pound and one shilling, an amount of money referred to by the British as one guinea (the gold used to make the coin coming from West Africa, an area known as Guinea). When other workers demanded more pay, the Italians continued to work for their original pay, hence the use of the word in a derogatory fashion.
Another possible explanation is that "guinea" once referred to the dark complexion of African slaves, and it eventually was used to refer to other dark-skinned people, such as the Italians.
Private Reiben makes reference to a "guinea" when he and other members of the squad are sorting dogtags and he comes across an Italian name.