To masturbate backwards- i.e. stroke from base to tip using both hands alternately. The move was made famous by Michael Jackson, who could also ejaculate in reverse.
At a party, people will often moonwank to show off.
"NO, that doesn't count as a moonwank. You're cheating by doing it in your socks/on the laminate floor."
During a full Moon a "Moonwanker" will stand by his window and Fap releasing a loud howl when he jizzes
Lee: "Oh look a full Moon"
JP: "What you doing tonight"
Sean: "Having a Moonwank"