An abbreviation of the name for the practice of 'tantric wanking'. This is when one masturbates to the point of near ejaculation, but then suddenly removes his hand from his penis and proceeds to sit on it in a meditatory stance, thus ceasing the self-stimulation. The wanker repeats this process over the course of two to three days. Needless to say the result is incredibly spectacular, but can unfortunately result in knob-ache.
Boy 1: Where have you been for the past week?
Boy 2: I was having a tank wank
Boy 1: woah.