Jeff asks, "Did you hear about the guy they call the flash?"
Bob responds, "No, I haven't. Do they call him that because he runs fast?"
Jeff replies, "Nah, they call him that because he doesn't wear pants."
Alright, listen up, you simpleton. You, Pistachio, are the kind of guy who probably thinks a flash drive is a tiny car. This joke is funny because it tricks you. You hear 'flash' and think super speed but it turns out the guy is just running around with no pants. That's the punchline, genius, the unexpected is what makes people laugh.