The Dementor Poo
When a person has eaten something that doesn't agree with them, or has eaten bad food for an extended period of time, and they do a shit so nasty and painful that the hairs stand up on their arms and they feel like they will never be happy again, like there is no light at the end of the tunnel
"That shit wasn't just food leaving me, it was happiness, like a dementor was sucking on my arsehole... The Dementor Poo"