To "bumper" refers to the act of collecting food left over by others after a meal in a restaurant (total strangers) and eating it yourself, without concern for potential infection. You do it after they've left but before the bus boy has cleaned up the table.
People have various "bumper lines," i.e., limits to which types of food or drink they're willing to bumper.
True "bumpering" is done regardless of whether the bumperer has money or is hungry. It's a principle-based practice to avoid wasting food. But hunger or poverty can also be major motivators.
"Dude, that couple's leaving behind a whole plate of fries!"
"Go bumper it bro!"
"Hey man, check out this sandwich I got."
"Is that a double burger with only one bite taken out?"
"Oh man, what an excellent bumper."
"You want some of this soda I bumpered?"
"No man, drinks are past my bumper lines."