As written by Remi Corson: http://www.remicorson.com/storing-woocommerce-terms-conditions-in-the-database/

 /**
 * Store terms and conditions value within the database
 **/
add_action('woocommerce_checkout_update_order_meta', 'woo_save_terms_and_conditions_status');
function woo_save_terms_and_conditions_status( $order_id ) {
    if ($_POST['terms']) update_post_meta( $order_id, '_terms', esc_attr($_POST['terms']));
}
add_action( 'woocommerce_admin_order_data_after_billing_address', 'woo_display_terms_and_conditions_status', 10, 1 );
 
function woo_display_terms_and_conditions_status($order){
	$terms = get_post_meta( $order->id, '_terms', true );
	$terms_status = ( $terms == 'on' ? __('accepted') : __('undefined') );
    echo '<p><strong>'.__('Terms & conditions').':</strong> ' . $terms_status . '</p>';
}