The ability to 'forget' something just when the moment may suit the person.
"Hey, Elijah, you going out with Teresa tonight like you planned all month?"
"No man, her Selective Forgetness happened to kick in and she didn't ask her mom"
"Sorry man, I know you were really looking forward to it"
"Ya, but I don't want to be pulling a Teresa
Just like a music instrument without frets someone would be call fretless by having no constrait, limitation, or restriction. Saying someone is fretless is a sexy and colourful way of saying he or she seems free in a cool way.