"imbahax" is a hybrid of the words "imba" and "hax". It is a paradox since hax is already imba. Saying, "imbahax", simply implies that someone is using hax or you sarcasticly say it to make someone feel like an idiot.
Using it to imply someone is using hax:
John: ZOMG, imbahax, htf did you kill our whole team?
Using it in a sarcastic manner to make someone look like a noob:
Player1: wtf how did you kill me so quickly?
Player2: LOLFACE, IMBAHAX