When a person means to pass gass but accidentally passes a small stool at the same time in their underpants. A blend of "fart" and "poo" and more kid appropriate than "shart."
When, on a particularly humid day, one tries to fart but a residual clot of defecation emerges from the anus with saidgas as an aerosol.
I was sitting in class and I tried to slip one out. I thought it was just gas but I should have known it was a farpoo in disguise. I've been spending the last ten minutes wiping mud out of my slacks
Short-hand for the word floccinaucinihilipilification, meaning the estimation of something as valueless (encountered mainly as an example of one of the longest words in the English language).