How to use Custom Field to display product brief description in shop page with Woocommerce?

The brief description of a WooCommerce product is store as post_excerpt, to programmatically add some text, you need to hook through product excerpt.

add_filter('get_the_excerpt', 'custom_excerpt');

function exc($custom_excerpt) {
    global $post;

    if($post->post_type == 'product'){
        $custom_add = __('This product is unavailable for some countries, please, read our terms & conditions to know more about this.', 'folkstone');

        return $excerpt . $custom_add;
    }else{
        return $excerpt;
    }
}