It’s an old Reddit story. Some sort of “what quirk does your family have, that you didn’t realize was weird” /r/AskReddit thread.

A dude’s entire family took shits so massive and solid that they had a butter knife that was designated as the poop knife. If they took a shit too big to fit down the drain, they would use the poop knife to dice it up before flushing. Apparently this was a fairly common occurrence, to the point that they just had it hanging on the laundry room wall for anyone to use in any of the bathrooms in the house.

And then he went to a house party, and had to take a shit. So he dropped a football in the host’s toilet, walked out of the bathroom, and casually asked where their poop knife was. Cue a literal record scratch moment, where the music was paused and the entire party turned to go “where is the what?” It was in that moment that the dude realized that normal people don’t have poop knives.

https://knowyourmeme.com/sensitive/memes/poop-knife

Origin

On January 9th, 2018, Reddit user LearnedButt[1] posted a thread to /r/confessions titled "[Light] I was 22 years old when I learned that not every family has a poop knife." In the story, the user tells about his family takes large poops, and had a knife dedicated for breaking up those poops when they wouldn't flush. When he was 22, he took a large poop at a friend's apartment and asked for a poop knife, leading to his embarrassment. Furthermore, he continued to have a poop knife in his house which his wife used, unknowingly, to open packages, thinking it was a utility knife. the post gained over 26,000 upvotes.

Precursor

On January 14th, 2014, Redditor[8] gobbluth69 posted a story regarding "poop scissors" in the /r/AskReddit subreddit. The post received more tahn [sic] 3,600 points in four years.

People used to cover their swords or knives in poop/shit so that non leathal scrapes or cuts would be sure to get infected at a time before medicine/antibiotics.