A really wet fart, so wet you might need a change of underwear. Normally has a slime like texture. Can be found in most older folks soiled undergarments.
"Grandpa left a little fart sauce in there for you."
1. often used as an insult for someone who is dimwitted.
2. greasy debris left from a particularly moist or raunchy fart.
1. shut your face, fart sauce!
2. I think that last one left behind a little fart sauce. That's gonna itch when it dries.