It's likely that the person using this term is referring to the mother of a person's father's sibling.
Here's why:
* Fraternal refers to siblings who share the same father but different mothers.
* Grandmother refers to a parent of a person's parent.
So, in this context, a "fraternal grandmother" would be the grandmother of a person's uncle or aunt, who is a sibling of their father.
However, it's important to note that this terminology is uncommon and could lead to confusion. It's generally more accurate to use terms like aunt or great-aunt when referring to a parent's sibling or their parent.