Generally when the word "keen" is used it is in the form of " I am not very keen on XXXX. The XXXX is the thing you are not very keen on. A few examples: I'm not very keen on baseball. - means you are not very interested in baseball. I am not very keen on cooking. - means you neither like cooking or are interested in it.

I am not excited or interested

I am not very interested

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It has alot of meanings depending on how it is used in a sentence. : Some examples that can relate to it are : "You're not very 'PRECISE/ACCURATE/SHARP WITH DETAIL' with something in particular".... (example) When it came down to keeping the house clean, she was not very keen", meaning she didn't always have the dishes done, laundry/clothes folded, floor swept, tables cleaned,etc.... Now if someone says exactly the phrase/expression "You are not very keen", then they usually mean that you are very inaccurate and sloppy with just about everything you do, you do not care how you do things, you don't take pride in your work/craftsmanship, etc.., you are incompetent of doing most things correctly, etc... It could also mean that "You are not very FOND of something in particular(meaning you don't really like or enjoy something)" Example : "I am not very keen on watching football", meaning I don't really like watching football. And literally one meaning of keen is "sharp", meaning "sharp" like a knife. Example : Be careful because that knife is very keen(very sharp), it will cut you very easily. Hope this helps you understand some of the meanings more. But here in the USA we hardly ever use that saying anymore, or at least I haven't heard it in years. We just say "you're sloppy with your work", "you can't do anything right", "I can't stand watching football", "be careful, that knife will cut you in a heartbeat",etc.... Hope this was helpful, Good luck