The Tuff Writer pen

The Tuff Writer assault tactical pen is essentially an extra-strength  shirt pocket-sized metal cylinder that you can deliver a vicious stab wound or six. And you can take notes with it too! In that you can actually use it write with it, The Tuff Writer technically qualifies as a writing instrument rather an out-and-out intentionally lethal weapon. Don't look for TSA agents, cops, court bailiffs, jail hacks correction officers, border guards, or greater than half-wit private security guards and such to agree with that notion though.

Even so the TW "pen" might be able to snuck into places where you couldn't go with a pocket knife, shirken, sap/black-jack, brass knuckles, pepper spray, or other more obvious weapons. Prices start at $80 USD.