slave driver

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An overseer of slaves at work.
• noun A severely exacting employer or supervisor.

from The Century Dictionary.

• noun An overseer of slaves at their work; hence, an exacting or cruel taskmaster.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An employer who demands excessive work from the employee.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a cruel employer who demands excessive work from the employees
• noun a supervisor of slaves at work