Wordorigins.org

This European name for part of the west coast of Africa has given rise to a number of terms and senses, but its origin is unknown. The name first appears in Portugeuse as Guiné, but beyond that we have no clue where it comes from.

It seems strange that a derogatory term for an Italian or Hispanic would come from the name for a region in Africa, but Guinea did not always carry this meaning. Dating back to the mid-18th century, the word was used to refer to blacks in the Americas. This citation from 1748 is recorded in the journal American Speech:

Run-away, a likely well-made Guiney Negro Man, named Toney.