A gaming lingo.
imba (short for imbalanced) is derived from Strategy games (e.g. Warcraft) or MMOs. It is usually used to describe when a player or a character gets way too powerful to defeat that it becomes unfair/impossible for the opponents to play against him. Which results in everybody (mostly the opponents) to give up. Almost invincible. It is mostly used as a praise rather than a complain.
When not used in context to a game, it means elite, awesome, the best or unbeatable.
with reference to a game:
"After getting some early-game kills, his character became imba. He pawned us all."
in general reference:
"The Joker in the Dark Knight is IMBA!! .. he definately deserves an Oscar."