Pulling and tugging on a flaccid penis in an attempt to get an erection.
Usually happens when you are just starting to masturbate.
It also happens when the person is unable to achieve erection, so they just keep tugging and yanking their penis hoping something happens.
Wife 1: I felt kinky the other day, so I went to my husband while he was asleep and pulled down his pants and starting jacking him off.... but...
Wife 2: But? What happened?
Wife 1: But nothing happened, it was like yanking taffy.