Source

File: wp-includes/taxonomy.php

function term_is_ancestor_of( $term1, $term2, $taxonomy ) {
	if ( ! isset( $term1->term_id ) ) {
		$term1 = get_term( $term1, $taxonomy );
	}
	if ( ! isset( $term2->parent ) ) {
		$term2 = get_term( $term2, $taxonomy );
	}

	if ( empty( $term1->term_id ) || empty( $term2->parent ) ) {
		return false;
	}
	if ( $term2->parent === $term1->term_id ) {
		return true;
	}

	return term_is_ancestor_of( $term1, get_term( $term2->parent, $taxonomy ), $taxonomy );
}