Currently on a few CoD games, it is a banned clantag for the consumer except the creators. The only way through getting it is hex editing one's clantag which is fairly easy.
IW stands for Infinity Ward. Also used for short to reference to Infinity Ward.
Online Friend:FUKING IW!!
Me: You mad bro? What's wrong?
Online Friend: I CAN'T USE IW AS MY CLANTAG!
Me: Here, I'll hex edit your CT.