1) Brilliant, but used in a mocking, snarky, or ironic sense.
2) Almost intelligent, almost smart, almost amazing... except not. So close to being brilliant, and yet so far away. Let's face it: just fail.
3) Someone who attempts to exhibit/emphasize their intellectual qualities or interests on the Internet. Used to gently poke fun at said person (usually "gently" anyway).
Alex: "Wow, that's pure genisu."
Harby: "Today, I managed to boil water without setting the kitchen on fire." :)
Mitch: "...Uh, yeah. Well done. You're a genisu, all right."
Nathan: "I'm going to name my RPG character after the forefather of the Carolingian Dynas--"
Joy: "Trying to look like a genisu, huh?"