Idears simple 'nuff- You just gotta say something that you think and/or hope there's no existing porn of, and then everybody else has gotta prove you wrong by either finding some, or making some. The first one to do so goes next.
Anyways, I'll start off easy:
Claymore. Never actually seen any porn of that. Maybe it just ain't that popular.
Just to clarify,
1 - The black thing on lower left is the PINGAS who assaulted me
2 - The grey-white bits are semen, since it's guro this can corrode skin
3 - I happen to be an alphabet string in real life!
4 - That hurts man, I bled gallons