A more thorough explanation:

Propatruus Magnus

Propatruus Magnus is a term used in Roman and civil law to refer to a great-great-great-uncle.

John's father's grandfather's brother is his propatruus magnus.

Samantha's mother's grandmother's brother is also her propatruus magnus.

These examples illustrate how propatruus magnus is used to refer to a great-great-great-uncle. In both examples, the person being referred to is the brother of an ancestor who is three generations removed from the individual.