The Collaborative International Dictionary of English v.0.48:

Taskmaster \Task"mas`ter\, n.
   One who imposes a task, or burdens another with labor; one
   whose duty is to assign tasks; an overseer. --Ex. i. 11.
   [1913 Webster]

         All is, if I have grace to use it so,
         As ever in my great Taskmaster's eye.    --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

taskmaster
    n 1: someone who imposes hard or continuous work

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "taskmaster":
   Big Brother, Simon Legree, auditor, boatswain, boss, chief,
   comptroller, controller, floor manager, floorman, floorwalker,
   foreman, gaffer, ganger, head, headman, inspector, monitor,
   noncommissioned officer, overman, overseer, proctor, sirdar,
   slave driver, straw boss, subforeman, super, superintendent,
   supervisor, surveyor, visitor