A more thorough explanation:

Definition: Abiaticus (ab-ee-ay-t-s), n. [Law Latin “descended from a grandfather”] refers to a grandson in the male line; a son's son. It is also spelled as aviaticus.

• John's father has a son named Peter. Peter has a son named Jack. In this case, Jack is John's abiaticus.
• When the king died, his abiaticus was next in line for the throne.

The examples illustrate that abiaticus refers to a grandson in the male line. In the first example, Jack is the son of Peter, who is the son of John's father. Therefore, Jack is John's abiaticus. In the second example, the term is used in a historical context to refer to the next in line for the throne, who is the king's grandson in the male line.