Its an infamous story on Reddit by u/LearnedButt, about a guy realizing that only his family had a specific knife dedicated to cutting poop when they are too big to flush. The original story had been deleted but an archived version can be found.

My family poops big. Maybe it's genetic, maybe it's our diet, but everyone births giant logs of crap. If anyone has laid a mega-poop, you know that sometimes it won't flush. It lays across the hole in the bottom of the bowl and the vortex of draining water merely gives it a spin as it mocks you. Growing up, this was a common enough occurrence that our family had a poop knife. It was an old rusty kitchen knife that hung on a nail in the laundry room, only to be used for that purpose. It was normal to walk through the hallway and have someone call out "hey, can you get me the poop knife"? I thought it was standard kit. You have your plunger, your toilet brush, and your poop knife. Fast forward to 22. It's been a day or two between poops and I'm over at my friend's house. My friend was the local dealer and always had 'guests' over, because you can't buy weed without sitting on your ass and sampling it for an hour. I excuse myself and lay a gigantic turd. I look down and see that it's a sideways one, so I crack the door and call out for my friend. He arrives and I ask him for his poop knife. "My what?" Your poop knife, I say. I need to use it. Please. "Wtf is a poop knife?" Obviously he has one, but maybe he calls it by a more delicate name. A fecal cleaver? A Dung divider? A guano glaive? I explain what it is I want and why I want it. He starts giggling. Then laughing. Then lots of people start laughing. It turns out, the music stopped and everyone heard my pleas through the door. It also turns out that none of them had poop knives, it was just my fucked up family with their fucked up bowels. FML. I told this to my wife last night, who was amused and horrified at the same time. It turns out that she did not know what a poop knife was and had been using the old rusty knife hanging in the utility closet as a basic utility knife. Thankfully she didn't cook with it, but used it to open Amazon boxes. She will be getting her own utility knife now.

[Edit: Common question - Why was this not in the bathroom instead of the laundry room? Answer. We only had one poop knife, and the laundry room was central to all three bathrooms. I have no idea why we didn't have three poop knives. All I know is that we didn't. We had the one. Possibly because my father was notoriously cheap about the weirdest things. So yes, we shared our poop knife.]

My friends sister laid a huge turd in the toilet. She couldn’t get it to flush. So she just casually strolls into kitchen while me and my buddy are watching Friday night videos, and grabs a pocket knife from the junk drawer. Goes back towards the bathroom, my buddy is hey what the eff you need a knife for? So she says the toilet won’t flush. Their dad hears this jumps up and runs down to bathroom and screams who the shit this turd. Which brings mom into the bathroom she freaks out. No all 5 of us are in the throne room in admiration of her magnificent turd. The dad asks what is the knife for and his sister goes it’s what I use to cut them. Yeah that’s what she said. It was silent until her mom asked how long have you been cutting turds with that knife, I am dying my friend is in tears, well since we are all here I guess the secret is out so I get massive turds on my period mom. So for about six years mom. She goes to leave and her dad grabs her and says cut the cheese sweet pea and that’s your knife now. That’s it I can’t breathe I am laughing so hard. She is in tears her mom is mortified and her dad was trying to be supportive, my buddy and I are being total jackasses.

Her dad pulls us aside and threatened to kick the shit out us if a word of sir turdly of bummertown gets out of the house. We never said a word about it outside of their house but we were brutal to here at home.