what is the meaning of the expression すげえまずい alone?
I've never read it before.
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すげえまずい <= すごいまずい <= すごくまずい i.e. gross/really awful tasting.
As for the change すごい => すげえ, you might be aware of, for example, いらない => いらねえ, both of which are very informal.
P.S. Since you provide no context, I assumed that まずい refers to food. まずい is used in other contexts, so you may have to adjust the translation accordingly.