lacking a logical or causal relation
"The sudden power outage was completely unrelated to the earlier storm that passed over our city."
not connected by kinship
"My cousin is entirely unrelated to our family tree because he adopted her after being found as a baby."
not connected or associated
"The two events were completely unrelated, occurring in different cities at different times."
In plain English: Unrelated means having no connection to something else at all.
"The incident turned out to be unrelated to his earlier mistakes."
The word unrelated is formed by adding the prefix un- to related to indicate a negative or opposite state. It entered English as a straightforward combination of these two existing elements to describe things that have no connection.