The amount of bin gravy varies with how lazy a household is: If you cannot be bothered to empty the dregs of your coke before chucking it or strain the milk off of your cereal you will create more gravy.... more lazy=more gravy.
Bin gravy is often a mixture of overly moist tea bag tea, takeaway drink dregs, cereal milk, decomposition juice from veg and meat, gone off eggs that have cracked, actual gravy, moistness from food left overs ..... etc
also could be referred to as garbage gravy by those speaking American (or overtly America-socialised people)
"shit, pass the antibac would you, there's bin gravy all inside the bin and I'm not putting a fresh bag on it...."
"will you PLEASE pour your milk into the silk..... I will not have you brewing cheesy bin gravy!!"