Usually it's telling someone to pick up a video game controler and play, it could mean pick up a lighter and hit a bong toke though, or if you to just tell someone to do something that they should already be doing, or having been told to do it already and lack the capacity to do so themselves without being yelled at.
Person 1 : "Do you wanna play video games?"
person 2 : "Yea, sure"
Person 1 : "Well then PICK UP THEM STICKS, BITCH!"
A person (usually one of your friends) that says they will do something with you, but then "stabs you in the back" and goes and does something else. (A "cunt kniver" is someone who will do this frequently with their girlfriend).
When someone you made plans with disappears and becomes a Knifer / Kniver