inbox rot 

To neither accept, nor decline a friend request from someone on Facebook or Myspace. Used in situations when you don't want to accept someone's friend request, but you also don't want to be rude by declining them.

Dude, that asshole friend of my girlfriend just sent me a friend request on Facebook. He's getting the inbox rot.

inbox rot by WHUDDDUP March 8, 2009

Inbox Rotting 

When one friends someone and instead of hitting accept or ignore, they press 'pending' and thus you are stuck in facebook limbo. The person can see updates of you on their news feed & vice versa, on profile picture updates, likes, friends, and posts to current friends. This can also be utilized by people who are bitches and instead of accepting a re-friend after a falling out, they leave the whole situation up in the air- in a creepy way where they can keep tabs on you.
You literally have your friend request left rotting in their inbox.

GUY: So she deleted every one she used to be friends with after that fight we all had- but I sent her a friend request after it all blew over... My mistake
FRIEND: what did she do?
GUY: Dumb bitch left me Inbox Rotting for like weeks!
FRIEND: Creepy, she was probably tracking you all that time!

Inbox Rotting by Typical... facebook May 20, 2011