Example: I’m no cook but I think that meat is raw. This means that the person talking is not an expert in that area but has an opinion they’d like to express (possibly to someone who is experienced. So ‘I’m no cook’ means that the person is not a cook, but in context, they are probably about criticise the food in the next sentence. Hope that makes sense :)

I just want to generalise Rohan's excellent answer. This type of phrasing can be used for any kind of trade or expertise. You'll commonly hear: - I'm not X but... here's my opinion. - I'm no electrician but, if I was you, I would get someone to have a look at that wiring. It is often used in a slightly sarcastic way as well. To preface the fact that you are about to state the obvious. Meaning you don't need specialist knowledge to know that it is wrong. For example; - I'm no expert but I don't think think the oven should be on fire. - I'm no plumber but I don't think water should shooting up from that tap. - I'm no cook but I'm pretty sure you're supposed to cook the meat before you serve it.

Takahashi san, the sentence may sound grammatically incorrect at first but it is actually ok if the “cook” being referred to is a noun meaning a person who cooks (usually as a job).. sou ieba, Nihongo de “watashi wa ryori o tsukuru hito ja nai” to iu imi desu..

It’s like saying I’m no expert but .....

Yes! It may mean that you want to share some info about cooking, but you are not a chef: I’m no cook, but this is too salty. Maybe someone is trying to be modest: I’m no cook, but I’m really proud of this pasta I made! Or ironic: I’m no political scientist, but I’m pretty sure Trump lost the election.