The act of fooling around and not doing your job. More specifically, the act of writing, or spraying, the numbers 40 on a wall in an undeniable reference to a 40 rock of cocaine. It's mostly used as an excuse for not doing your work.
Person 1 "Are you slacking off again?"
Person 2 "Nah man, I'm testing spray patterns!"
Person 1 "Why are you writing 40 everywhere?"
Person 2 "I was just testing spray patterns."