As for the possibility racist origin, it should also be mentioned that the term was used to describe the male of various species long before Africans were subjected to slavery by Europeans.

Amer Her:buck 1 (bîk) n. 1. a. The adult male of some animals, such as the deer, antelope, or rabbit. b. Antelope considered as a group: a herd of buck.

Regards,

A.