It's someone who helps another person who has an eating disorder, to lose weight. They are fetishists most of the time, and will act predatory toward and fetishize girls with an eating disorder.
(noun)-An individual who provides advice and support for someone who has to urgently take a massive shit with the intention of helping them to not take a massive and steamy dump in their pants. Truly one of the most heroic actions one could take.
Heather: John, drive faster, I really need to poop, I might not make it!
John: It's okay, I'll be your poop coach. What you need to do is pucker as hard as you can, remember to breathe, and visualize clogged pipes, corked bottles, and blocked holes.
Heather: Thank you John, you're the sweetest husband ever.