The phrase "slave driver" refers to someone who is very strict and demands a lot of work from others, often without considering their well-being or feelings. Full definition
If your «workout slave driver» voice is drowning out your «soulful loving movement» voice (the voice that says, «take a chill pill, you already went spinning four times this week»), read this to redefine your relationship with exercise and start to make peace with your body once and for all... (mindbodygreen.com)
No big villain, he's evil's middle management, a vile slave driver who just wants to complete the mission and leave this Pacific hell hole behind. (startribune.com)