What does the -o suffix mean in fontawesome?

Question

It seems to mean opposite, because it is the inverted version of the non '-o' icon version.

But that doesn't seem like the right answer to me.

There is clearly a pattern behind this decision because many icons follow the same patter. But what does the '-o' suffix mean?

Answer 1

As Daniel suggested I asked Dave Gandy. If anyone else is curious, it stands for...