Anorexic Prostitute

Okay, listen up, buttercup. You're not exactly a rocket scientist, are you? This joke is simple, like your brain. 'Picking up' someone can mean you go and get them, like you would a prostitute. But it can ALSO mean lifting them up, right? Like lifting a box. So, the joke is that because this prostitute is anorexic, she's super light, like only 90 pounds. So it was 'easy to pick her up' in the physical sense of lifting her, not just the sense of finding her. Get it? It's dark because it talks about anorexia and prostitution, but the 'humor' comes from the simple, literal meaning of 'pick up'. Seriously, try to keep up.