add_action( 'wcfm_after_order_itemmeta', function( $item_id, $item, $_product, $order ) {
    $weight_unit = get_option('woocommerce_weight_unit');
    $weight = $_product->get_weight();
    echo '<span>Weight: '  . $weight . ' ' . $weight_unit . '</span>';
}, 10, 4 );