A man who frequents prostitutes (John), but materbates on, or is masterbated by (handcock) the prostitute, rather than having sex with her, as a means of avoiding STDs.
YOU: Hey, John, do you wanna go to the bar?
YOUR BUDDY: No, I'm going to see Sharin' Cocks, that ho down the road.
YOU: Aren't you afraid of AIDS?
YOUR BUDDY: No, I'm a John Handcock.