You know when you're jerking off and cycling through all those chick in your mind... well the finisher is the chick you finish jerking with.

A girl who makes her partner cum entirely with her hand or mouth. Not like too many porn scenes where the guy ends up masturbating himself to ejaculation.

When you're jerking off and cycling through all those ladies in your mind, The Finisher is the girl that you climax to.

To achieve orgasm

said to encourage complete debasement of opponent in a battle of words

Meaning: It's over and done with. Whether it's a relationship or a book coming to an end. No longer necessary.