slave driver

American Heritage® Dictionary of the English Language, Fourth Edition

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

Century Dictionary and Cyclopedia

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

Wiktionary

1. n. An employer who demands excessive work from the employee.

GNU Webster's 1913

1. n. one who superintends slaves at their work; hence, figuratively, a cruel taskmaster.

WordNet 3.0

1. n. a cruel employer who demands excessive work from the employees
2. n. a supervisor of slaves at work