<meta name="description" content="where this comes from?">
The tag itself is output by the base theme:
So in order to override this value you could either replace the value of $this->content['description'] in the initialize() function of your custom theme or override the whole head_metas() function there too.
Currently the value is set based on the initial characters of the content of the question (if in the context of the question) or based on the home description (if in the home page):
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