What exactly makes a knife a "penis knife"?

It's Monday...no wait, FAHK, it's Tuesday.  So it's Tuesday, and I would like to say that I have been so wicked productive y'all would be proud.  So far this week, I have:

1. laid in bed and groaned in pain Uhhhhh finished 3 homework assignments for school
2. cried about the weather like a little ole lady talked to Stomp about being a kind, thoughtful human being
3. brought sexy back to limping taught Boston all about who really cut John Wayne's pee-pee off
4. slept  Yes, y'all, THAT John Wayne
5. watched way too many episodes of the Blacklist  Debated the merits of what is an appropriate penis knife