Zippermouth

1. Describes someone who you really wish would shut up for once

2. Describes someone who excels at keeping secrets and being quiet when they need to be.

(Definition 1)
Dave: Man, Randy really won't shut up will he? He shares way too much information!

Bill: Yeah, he's such a Zippermouth

(Definition 2)
Craig: Are you sure I can trust you with this?

Daniel: Of course, I'm quiet as a Zippermouth, nothing comes out of here unless I want it to.