Racist term from the old South. While entertaining in the summer, after dinner the good White folk would retreat to their respective spaces (women to the parlour or drawing room, men to the porch) for after dinner drinks. The men, desiring to prop their feet up while relaxing, would have the small slave children get on their hands and knees so that the master and his guests could use them as footstools. Hence the term "porch monkey".
"Delilah, go and fetch those little porch monkeys so that my company can rest their feet a spell".