I can't seem to find any documentation that officially comments on this tag. Maybe a few forum posts here or there, but nothing on Webmaster Guidelines and even here on Pro Webmasters, I'm not seeing anything relating to it.
I encountered this as I was sifting through links to un-do some rather horrible SEO choices by some former SEO firms our company contracted with.
My experience tells me that
...rel="nofollow external">anchor text</a>
is just a way for the site owner to keep our links while making it seem like he's trying to help us? I contacted him about a week ago and we're about to go through our second round of emails. Since he never contacted me back, I'm likely going to try and get a hold of him again. I honestly don't remember if it had this attribute before I messaged him last time around, but I'm fairly certain it didn't.
So, in a nut shell, my question is: will this actually remove the link from Google's radar or do I need to contact this guy again?
EDIT: Just so it's clear, I understand
rel="nofollow" is happy and wonderful, it's the external bit I've never seen before and I know people can make up their own meta tags to add stuff to the code that never gets used, like a KEYPHRASES meta tag.