A play off the word "undead". While these energized corpses are neither dead nor alive, an unslut is a girl who is neither a slut nor chaste. She thinks and acts like she is a slut, yet lacks the physical features to actually arouse even the horniest, most hormonally driven men.
So this girl was all over me at the party, even though she had a boyfriend, which would've been fine except that I've had teeth fillings that have given me bigger erections. She was a huge unslut.