basic wordpress api endpoint to serve a key-value dictionary

You could create your own WP REST API endpoint that handles this. To register a custom endpoint:

add_action( 'init', 'bootstrap_api' );

function bootstrap_api() {
    add_action( 'rest_api_init', 'my_api_endpoint' );
}

function my_api_endpoint() {
    register_rest_route('myapi/v1', 'get/words', array(
        'methods' => 'GET',
        'callback' => 'return_word_list',
    ) );
}

function return_word_list() {
    // Do someting to build your list of words...
    // $words = get_my_words();
    $words = array(
        'key' => 'value',
        'foo' => 'bar',
        'baz' => 'blah',
        // etc
    );

    return $words;
}

Then, when you visit /wp-json/myapi/v1/get/words, you’ll see something like this:

{
    "key": "value",
    "foo": "bar",
    "baz": "blah"
}

This is quite barebones, and I’m not sure what you’ll need to do in terms of the Jetpack JSON stuff for authentication, but it should get you off on the right path.