One experiences cockmouth when they are near or around a guy (or guys) and want to impress them with either their bullshit lies or actual awesome stories. Either way, when you are being a cockmouth, you just sound like a bragging bitch. A tell tale sign that you are suffering from cockmouth is when the people around you cannot stop rolling their eyes.
Julia: Yeah, Brad, I totally bungee jumped off of that bridge in Sydney, right after I explored the reef and fended off a giant sting ray who was trying to mate with my bestie. Did I tell you, I'm a trained animal psychiatrist, oh, and that stingray thing totes made me a local celebrity.
Sarah: (rolls eyes) Wow, Julia just won't shut her big fucking cockmouth.