Bonch is a term used to describe the liquid that collects in the bottom of a full garbage bag. Used by those who work in the food service, parks and recreation, custodial services, or other such industries where changing out garbage bags is a common task. Also used with modifiers to describe the main constituent of the liquid (see below).
When I was changing out trash cans at the waterfront, I tore a hole in a bag and dripped bonch all over my pants.
Get that sack of empties out of here! I don't want beer-bonch leaking all over the carpet!