1) When a girl has been with a guy so long that, even though he is now a total doosh, she stays with him out of loyalty, and fear of being alone.
2) A pickup technique whereby you force yourself onto a girl for so long she starts to enjoy your company, and eventually WANTS to have you around.
"Karl is such a tosser! Why is Steph still with him?"
"Classic cockholm syndrome..."
" Yes! Jill just called me! My persistance has finally paid off!"
"Mate, you fully gave her cockholm syndrome. Score!"