slave driver

1. An overseer of slaves at work.
2. A severely exacting employer or supervisor.

There was no ignoring slavery in early nineteenth-century America. In the North it was dying out, and abolitionists were beginning to speak against it; but in the South slavery was more profitable than ever, thanks to the growth of cotton plantations worked by slave labor. In 1807 the British abolished the import-export slave trade, and in 1808, after a twenty-year wait imposed by Article 1, section 9 of the Constitution, the United States did too, but the number of native-born slaves continued to increase and restrictions on them grew harsher.

As if to emphasize the inhumanity of slavery just as there were drivers of cattle, so there were drivers of slaves, a word used as early as 1763 for those who literally whipped slaves to work. The combination slave driver, oddly enough, appears first in an 1807 satire by Washington Irving that seems to have nothing to do with American slavery. It is a purported letter "to Asem Hacchem, Principal Slave-Driver to His Highness the Bashaw of Tripoli" from his American agent. The letter directs its satire at the liberated women of New York City, not at slavery, but the ascription of "Slave-Driver" to a notorious realm of pirates implies a recognition of its barbarity.

It was an epithet to provoke a fight. In 1856, during the battle for "bleeding Kansas" between pro- and antislavery settlers, an abolitionist prayer in a Maryland newspaper asked, "Our Heavenly Father...help us to shiver the Union into atoms rather than to concede to Southern demons, in the form of slave-drivers, one inch of the disputed territory."

Slavery fortunately is long gone, but the slave driver lives on today in the form of the merciless taskmaster. And as Henry David Thoreau points out in the first chapter of Walden (1846), this person is not necessarily an employer: "It is hard to have a southern overseer; it is worse to have a northern one; but worst of all when you are the slave-driver of yourself."

The noun has 2 meanings:

Meaning #1: a cruel employer who demands excessive work from the employees
  Synonym: Simon Legree

Meaning #2: a supervisor of slaves at work