Translation:It is not good.
The outdated way of reading 良い is えい. Also bear in mind that most (if not all) common Japanese words are usually written in Hiragana only. Here is certainly the case.
I don't know why it became いい from えい, but my guess is for the sake of ease of speaking. It already happens as a syntactic rule when か turns into が, た into だ, etc.
For instance 時時 or also written as 時々 spells ときどき or 神奈川 prefecture spells かながわ where 川 is changed from かわ to がわ.
Or as common pronunciation when for example you drop the 'u' sound in conversation when using です or ます and say 'des', 'mas'.
I think good is "yoi" and to make it negative you have to take off the 'I' and replace it with 'kunai'. We know it does not mean 'often' because there is a 'nai' after.
Furthermore "often" is not an adjective so you wouldn't use "kunai" after it- plus that would make it "yokukunai".
Spelling (written form) and context (spoken + written form.) But since Japanese commonspeech tends to omit a lot of stuff, I can see where confusion would come into play here, especially since "good" and "often" both describe things.
Does anyone know if the kanji for "often" yoku is different from the kanji for "not good" yoku?
it's the same word for both, 良く is more about frequency, the how often does something happens indicates your ability on that stuff, so for example if you say「よく料理する」you are saying that you cook so often that you are already good at it, this is less pretentious than using 上手 in Japanese culture.
So the meaning of よくない "not good" comes from that inference, even to describe things that are not good, if you describe food as よくない is more like saying that is not skillfully done than directly bad.
Thank you for the response! Your explanation really helped me understand the connection between the yoku meanings. :)
Bonus: Had to look up jouzu since I didn't recognize it in kanji form. I kept spelling it "uete." XD Interesting how it means "up hand." Like a high-five!