To masturbate only by imagination. Without pornography or live action sexual actions.
After catching a glimpse up Kelly's skirt in class Jim fanterbated about it alone in his closet.
When you have to masturbate based purely on your imagination, with no porn of any kind. Often used by 12 year olds in dark rooms who have no access to porn, a raging boner, and only the searing memory of seeing up a girl's skirt in school that day.
theac fanterbates to BVA