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1.
The liquid run-off that gathers at the bottom of a bin bag from all of the squelchy things that you have put into your bin.

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)
"oh my god!! Could you check for splits before you take the bin out?! you've left a trail of bin gravy all over the floor!"

"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!!"
by ciboruggle September 18, 2012