Now I feel like I'm slacking ;)
I've always assumed 'imba' was short for 'imbalanced', as in too powerful. As in 'OMG NERF ROGUES THEY ARE IMBA!!!11'. Meaning your voice is too good ;)
I'll have a better version up this weekend, and I'll post a link here to download the .mp3 when it is done.
I'm warning you though, my singing doesn't get any better.