“copyleft”

Everybody knows what this symbol  © means.  It’s that type of symbol that says, “Don’t mess with me, unless I allowed you to.”  But if there’s such a word as “copyright,” is there an antonym of the word similar to right having its opposite, left?

I was reading a blog from Mashable about “The Last True Hacker” when I chanced upon “copyleft.”  The word does exist but not in Microsoft Word’s vocabulary.

And logically, it's symbol looks like this.  No kidding!