Imba is used mostly in online games or games that have patches or different versions. Imba means that something is imbalanced and should be "nerfed" or made worse. When something is imba it means that is is too good and it makes the game unfair.
Richard: I'm not surprised you killed me Steasl Assasin is so imba.
Tod: Yeah they should nerf him in the next version he's to good.