Urine that is left to settle in a toilet bowl. Comes from the energy saving saying that goes "if its yellow, let it mellow. if its brown, flush it down."
Not to be confused with Mellow Yellow which was a song by Donovan in the 60's composed when he took the drug Mellow Yellow. Also not to be confused with the drink popularized by Coca Cola.
"Stop leaving yellow mellow in the toilet! The dog drank out of it and licked my face!"