A person who often has sex out of charity or pity in spite of having insufficientsexual attraction. The not-for-profitute often feels like they ought to have been payed, but refuse to receive payment as their sexual performance is a gift of charity.
"Kev, dude. Your face is glowing; what's good man?"
"Man, I dicked Brittney last night. She's like a 10. I'm like sexy and I never even knew."
"I hate to break this to you Kev, but Brittney's a not-for-profitute."
"Oh. Dude, I'm not even mad though. Her tits were bouncin'."
A more polite way of saying "slut"; remember that a whore gets paid while a slut does the same work for free.
From SNL parody of Girls:
Jessa: I had sex with a cab driver, alright? It's called being free.
Blerta: You are prostitute?
Jessa: No, it's not prostitution if you don't get paid.
Blerta: You are UNPAID prostitute?!? You are lower than dog!