It's in reference to Caps Lock. Cap. Slock, or Captain Slock. Some say he's a pirate but that has yet to be proven.
DO YOU DOUBT ME AND THE POWER OF CAPTAIN SLOCK?!?
A Captain Slock Is when you pretend you are going to stick your penis into a womans mouth and instead dick slap them real fucking hard
Dude:"i gave my girlfriend a Captain Slock yesterday all you heard was that loudass smack"
Dude's Friend: "What did she do?"
Dude: "She tried to slap me with her vagina flaps"