The context you've quoted it in doesn't seem to make sense, but in English all of these 'you're one xyz short of a....' are essentially all the same. They usually mean that someone is crazy, or that something doesn't work properly.

You can see more idioms on my website.

The context you've quoted it in doesn't seem to make sense, but in English all of these 'you're one xyz short of a....' are essentially all the same. They usually mean that someone is crazy, or that something doesn't work properly.