add_filter( 'woocommerce_short_description', 'wpcb_add_read_more_to_short_description' );
if ( ! function_exists( 'wpcb_add_read_more_to_short_description' ) ) {
    /**
     * wpcb_add_read_more_to_short_description.
     */
    function wpcb_add_read_more_to_short_description( $short_description ) {
        $words     = explode( ' ', $short_description );
        $num_words = 55;
        return ( sizeof( $words ) > $num_words ?
            implode( ' ', array_slice( $words, 0, $num_words ) ) .
                '<details><summary>Read more...</summary>' .
                    implode( ' ', array_slice( $words, $num_words, sizeof( $words ) ) ) . '</details>' :
            $short_description );
    }
}