According to the Jibbigo JP mobile app, it means "I would (do that)". I tried Japanese Wikipedia, bing.com and my translation software, but no results.
Here's an example:
A girl says something like this,
なんでわかってくれないかなぁ、この気持ち!いいーーーっだ!
what would the "いいーーーっだ!" mean?
Maybe you guys can figure something out.