Changing the default wp_search_stopwords

You are close: you indeed need to define a filter, but you’ve not quite gotten the filter definition right:

function add_meh_stopword($stopwords) {
    $stopwords[] = 'meh'; // *add* "meh" to the list of stopwords
    // if instead you want to completely replace the list of stopwords
    // use $stopwords = array('meh');
    return $stopwords;
}
add_filter( 'wp_search_stopwords', 'add_meh_stopword');

The second argument to the add_filter function takes a “callable”, i.e. in this case a function name. That function then returns the modified list of stopwords.

Hope this helps!