Great-uncle is a term used to refer to one's grandfather's sibling. However, there exist several synonyms to this term. The first one is "granduncle," which is a combination of the words grand and uncle. The second synonym is "gruncle," which is derived from the words "great" and "uncle." "Grunkle" is a slang term that can also be used to refer to one's great-uncle. Another term widely used is "grandfather's brother." Lastly, the term "uncle once removed" is another way to refer to one's great-uncle, but it can be a bit confusing since it can also refer to one's cousin's child. However, all these synonyms mean the same thing, which is the sibling of one's grandparent.