I dont know the EXACT reason, but from what ive heard some people were abuseing the term. The Admins block it now.
There was an entire topic about it at one point. Everyone is wondering how you could possibly abuse that term enough, to make the system prevent you from posting it. Just like **** and ***** its an auto filter.
Your message has met the filter designed to stop ultimately silly messages, hit the back button to edit it and repost. This message was triggered by using words longer than 50 characters, the term m o nk item in your post, or spamming.