The practice of numbing one's action hand, generally through circulation restriction or ice application, for the purpose of pseudo-auto-erotic masturbation.
I.e., it feels like someone else is doing it. Popular extensions include shaving and manicuring the hand to give a visual as well as sensory illusion.
So called due to the increasingly eccentric and imaginative masturbation aids that long periods at sea causes.
Seaman Staines: Ahoy! Why is there no ice in the freezer?
Master Bates: Because I just used it all to numb my hand and rub out a cheeky Sailor's wank all over your insoles.
Captain Jack Sparrow: Ahar, me matey. Where's me rubber bands for me hair?
Roger the Cabin Boy: Sup Jack. I took your hair bands for an extravagant Sailor's wank on the poop deck. I can offer you this delicious, creamy white conditioner in return?