Simply put. A friend that doesn't belong inside.
Criteria for an Outdoor friend:
1. Doesn't readily learn the rules of the inside. ( i.e. don't leave the swisher guts on my table.)
2. Is happier outside (or at least makes everyone else happier when they are outside)
3. Misses the toilet
4. Clogs the toilet
5. Easy to trick/tease/torment etc.
Nick: Why do i have to be outside?
Me: Because you are an outdoor friend.