The dead end one experiences on finding that an attractive woman friend of a friend has no photos, or high privacy settings on their facebook profile. From "Cul-de-sac" - dead end road and "Fap" to enjoy oneself too much looking at porn.
"Oooh, look at her… I wonder if she has any hot Ibiza holiday pics… NOOOOOOOOOOOO!!! Cul-de-fap! :((("