A list that guys can keep of girls who are just friends, and who nothing more would ever EVER happen with.
Girls can also have 'safe lists' which would (due to girls infinitely pickier nature!) be a lot longer than those of their man-whore guy mates.
Fiona: hey Chris, whats with you and Kathryn going camping together? Is something going on between you two?
Chris: no way! She's on my safe list.