Just pushing the stupidity meter of definitions that UD will publish. If by chance this gets published, and you're unfortunate enough to stumble upon this while looking for an actual definition, move on, my child.
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!"
OR
Person 1 "Why are you writing 40 everywhere?"
Person 2 "I was just testing spray patterns."
This is a term that me and my friends refer to when asking if someone has shit stains in their underwear. These are not the lightly browned streaks, but the heavy streaks that look as if someone was using the underwear to help heat up their racing tires at the track.