A render bender is when you are rendering something on a computer that is going to take a long time, usually 2 days or more. Often used in reference to motion graphics, video editing, 3D modelling and games design.
"Craig wanna come out for a few beers tonight?"
"Nah mate, got stay at home, I'm having a render bender..."
"Alright, fuckoff then you geeky shit!"