Answer:
Your great-n uncle is either the brother of your great-(n-1) grandparent, OR the wife of the sister of your great-(n-1) grandparent, where 'n' represents the number of 'greats', e.g. your great great great great uncle (n=4) is the brother of your great-great-great grandparent or the husband of the sister of your great-great-great grandparent (i.e. your great great great great aunt's husband).