Bad Tasting
Unappetizing, Unskillful
い adjective
You've seen 美味しい by this point. This is the opposite of that. Instead of 美 you have 不 (not), so something that does not have flavor is something that doesn't taste good. i.e. something bad tasting. This can also refer to someone who's unskillful at something as well.
The reading doesn't follow any of the rules you've run into. It's まずい. Think about how you're going through this really long maze (まず) and how you're promised a really tasty wonderful treat at the end for getting through it (you're a lab rat or something). When you get to the end, there's your treat! What joy! You eat it and... oh man... it is so bad tasting you can't even believe it. Be sure to imagine the terrible taste and the terrible disappointment after going through that difficult maze only to get this... this... terribly unappetizing cheese.
なんか不味そうなサンドイッチだね。
Those sandwiches look a bit unappetizing, don't they?
あの字はちょっと不味いんじゃない？
His handwriting is kinda bad, don't you think?
こんなに不味い味噌汁を飲んだことはありません。
I’ve never eaten such disgusting miso soup.