I know the meanings of gang/prison tattoos.
The other day my roommate and I were out shopping (at Walmart, where else?) when we passed a guy with several tattoos.
"Omiword!" I whispered, "That guy has totally killed someone!"
"What?! How do you know?" she asked.
"Um, look at that tattoo. You only get that if you've whacked someone!"
She didn't entirely believe me until we went home and looked it up on Wikipedia (ha! nerds). When I was proven correct, she awed at my extensive street knowledge. I didn't mention that the only reason I know this stuff is because I've seen Con Air like 20 times.
But still—valuable information to have, amiright?