A pen moocher is someone who begs for pens from somebody who had just bought a new pack of pens even though they don't really need one at the moment.
Katherine: I had to buy some more pens since I keep losing them at home somehow.
Liz: Can I have one?
Katherine: Do you really need one?
Liz: I might.
Katherine: What happened to the pen I gave you two days ago?
Liz: This one might be better though
Katherine: Such a pen moocher