Seemed have found a solution via customizing a code snippet from here:
http://wordpress.org/support/topic/get_the_terms-using-array-of-taxonomies
$taxonomy = get_the_terms($post->id, get_post_meta($post->ID,'metafield-taxonomy-name',TRUE) );
$terms = get_the_terms($post->id, $taxonomy );
- $taxonomy must be placed outside of post loop.
Thanks!