I'm reading a manga and I can't for the life of me figure out what this phrase is supposed to mean; I've never seen it before.
This looks like a slurred or lisped version of すごい ("awesome").
Slurred or lisped pronunciation is often marked in Japanese by using different kana. In the example above, す is swapped out for しゅ. Years ago, my wife had a DoCoMo phone that had an animated bear that would appear and say various things in a funny lisped accent, where all the "s" sounds were swapped out for "f" sounds. The bear would say things like げんき でふぉう, or とても ふき でふ！
Please comment if the above does not address your question.