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.
A theoretical act of self pleasure, often described as masturbation squared, or F=M^2. Can also describe intense masturbation, or simple everyday masturbation for hyperbolic effect. The word is simply a contraction of the slang term 'fap' and the word 'masturbation'.
"My parents just caught me fapsturbating to skinemax, no tv for a week."
"Man, that's harsh. They always say fapsturbation is a sin, but we know everyone does it."