Invi is short for Invictus.
Invictus is Latin for invincible, unconquerable, etc.
It is also the name of one of my favorite poems
, written by William Ernest Henley.
I used to use Invictus a lot, but it is usually taken now. Some friends in a chat started calling me Invi a while back. I like the sound of it, and it still retains meaning to me, so I continue to use it. It's taken less often. heh.
I like Invictus because of the idea in the meaning of the word.