A term popularized by the video-sharing site youtube.
This term means that somebody/someone or something isn't working, or functioning to their full capacity.
This can be used as a synonym for MIA on aim or msn.
1) www.youtube.com: "we're updating now, but we'll be 100% back in a bit"
2) Bill: Dude, are you OK? You don't seem yourself..
Bob: Yeah, I've kinda been not 100% this week, ya know?
3) user1: hey ima b not 100% 4 a bit
user2: its all good