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."