Flatulen
The poo particles that are released as you fart, these flatulen particles make the air stink and can be left on your underwear.
"I had such bad flatulence, the flatulen left residue on my underwear."
"Dude, you must have flatulen on your underwear, you smell like straight ass"
"Dude, you must have flatulen on your underwear, you smell like straight ass"