Origin and history of guinea

rodent native to South America, 1660s. It does not come from Guinea and has nothing to do with the pig. Perhaps so called either because it was brought back to Britain aboard Guinea-men, ships that plied the triangle trade between England, Guinea, and South America [Barnhart, Klein], or from its resemblance to the young of the Guinea-hog "river pig" [OED], or from confusion of Guinea with the South American region of Guyana (but OED is against this). Pig probably for its grunting noises. In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in medical experiments (by 1865).

by 1550s in reference to the North American bird, from the place name Turkey, likely in its vague 16th-century sense of "Asia" or "Ottoman lands." The bird seems to have become known in England via Ottoman North Africa or in part because the New World still was not distinguished from Asia.

It is by 1540s in English in reference to the guinea fowl (Numida meleagris), a bird imported from Madagascar via Turkey, and called guinea fowl when brought by Portuguese traders from West Africa (compare guinea).

The larger North American bird (Meleagris gallopavo) was domesticated by the Aztecs, introduced to Spain by conquistadors (1523) and thence to wider Europe. The word turkey was applied to it in English 1550s because it was identified with or treated as a species of the guinea fowl, and/or because it got to the rest of Europe from Spain by way of North Africa, then under Ottoman (Turkish) rule. Continuing 16c. confusion in the popular mind of the New World, Asia, and the Indies might have contrbuted. Indian corn was originally turkey corn or turkey wheat in English for the same reason.

The Turkish name for it is hindi, literally "Indian," probably influenced by French dinde (c. 1600, contracted from poulet d'inde, literally "chicken from India," Modern French dindon), based on the then-common misconception that the New World was eastern Asia.

After the two birds were distinguished and the names differentiated, turkey was erroneously retained for the American bird, instead of the African. From the same imperfect knowledge and confusion Melagris, the ancient name of the African fowl, was unfortunately adopted by Linnæus as the generic name of the American bird. [OED, 1989]

The New World bird itself reputedly reached England by 1524 at the earliest estimate, though a date in the 1530s seems more likely. By 1573, turkey was becoming a usual main course at an English Christmas. The wild turkey, the North American form of the bird, was so called from 1610s. 

The meaning "inferior show, failure," is attested from 1927 in show business slang (Vanity Fair), probably from the bird's reputation for stupidity. The meaning "stupid, ineffectual person" is recorded from 1951.