Green’s Dictionary of Slang

guinea n.1

[18C guinea, a black. The original guineas were black slaves from the Guinea Coast of Africa, and the term gradually evolved to mean anyone with a notably dark complexion, although it is rarely if ever used to describe a black person in 20C+ other than in the SE Guinea Negro, a mixed-race group native to Maryland, Virginia and West Virginia, who call themselves Our People or Melungeons]
(US)

1. [late 19C+] (also ghini, ghinny, gin, ginee, ginney, ginny, guin, guiney, guinney, guinny, guiny, guinzo, guinzola) an Italian person, usu. an immigrant to the US.

2. [1910s+] a derog. term for various other non-Anglo nationalities, usu. Mediterranean, e.g. a Greek, a Portuguese, a Jew.

3. [1900s] (US Und.) a woman.

4. [1910s+] (also ginney) a foolish, gullible or insignificant man.

5. [1940s] a Japanese person, a Pacific native.

6. [1950s] (also ginny) a groom in a horseracing stable [US grooms were often of Italian immigrant origins].

7. [1950s–60s] the Italian language.

In phrases

get someone’s guinea up (v.)

[1930s] to make someone angry.