A "stable buck" is the lowest job in the stable. This is the
person who'll do most of the heavy lifting and most odious jobs.
The most famous example of one is probably Crooks from "Of Mice and
Men". There is an argument to be made for "buck" to be a reference
to his race, but an as compelling one for this "buck" to be derived
from the military custom of using "buck" to denote the lowest form
of any rank: buck sergeant, buck private. Through shortening, this
becomes a person of no status other than male.